You typed a date into a box and a machine told you when your baby will be born. It feels like magic, but it is arithmetic — and once you understand the arithmetic, you understand something far more useful than a single date on a calendar. You understand how doctors count, why your app and your midwife sometimes disagree by five days, why a due date is a median rather than a promise, and what actually happens inside the 280 days the number represents.
This guide walks through everything our pregnancy and due date calculator does, the exact formulas behind each of its four dating methods, and the medical reasoning that makes those formulas the accepted standard. It includes complete quick-reference tables — week-by-week fetal length and weight in both metric and imperial units, unit conversion formulas, cycle-length adjustment offsets, official trimester day boundaries, the professional criteria for redating a pregnancy by ultrasound, and a fourteen-point prenatal timeline. Whether you are two weeks past a missed period or thirty-six weeks in and packing a hospital bag, there is something here you did not know.
Pregnancy Due Date Calculator Embed Code for Your Website:
You have just read through the full architecture of this tool — the four distinct dating methods that handle LMP with cycle‑length and luteal‑phase adjustments, ovulation tracking, IVF transfers with day‑3 and day‑5 anchors, and ultrasound back‑calculation; the coherent internal calendar that derives gestational age, days remaining, trimester boundaries, and the fertile window from a single reference point; the week‑by‑week size tables that track length and weight in both metric and imperial units; the fourteen dated milestones that turn vague screening windows into calendar dates your visitors can actually act on; the exact formulas laid bare with no black boxes; and the quiet insistence on zero ads, zero sign‑ups, and zero data collection. You have seen the cycle‑length correction table that shifts a due date by up to seventeen days, the luteal‑phase variable that most calculators never ask about, the Naegele's rule history and its modern alternatives, and the honest explanation of why a due date is a median rather than a promise. Now comes the practical part: putting that same engine onto your own site, so your visitors can calculate their due date, track their progress, and plan their prenatal timeline without ever leaving your pages.
You can copy the code below and embed it on your website — in articles, sidebars, footers, or anywhere you want visitors to access a clinically grounded due date calculator. The tool runs entirely in the browser, inside an isolated container that cannot read or touch the page around it. No personal data — no LMP dates, no cycle lengths, no ovulation dates, no ultrasound measurements — is transmitted, stored, or logged. All calculations happen locally on your visitor's own device. The four dating methods, the cycle adjustments, the trimester boundaries, the milestone conversions, and the week‑by‑week reference tables all work without a single server round‑trip. Close the tab and the entire session evaporates. What your visitors calculate stays with them, which is exactly how a pregnancy calculator should work — it is not the due date that matters, but the clarity and confidence the numbers give to the person reading them.
The embed is available in six interface languages to match your audience: English - Spanish - French - Arabic - Chinese - Hindi. The language setting controls the tool's labels, buttons, and instructions — and also determines which language the results, the week‑by‑week descriptions, and the milestone explanations appear in. The calculator itself is the same for every visitor worldwide, but the language it displays in is entirely your visitor's choice, detected automatically from their browser on first visit and remembered afterward. For Arabic, the entire layout flips to right‑to‑left, mirroring the interface so the experience feels native.
Switch the interface language by changing the parameter in the URL: ?lang=en
Available languages: en - es - fr - ar - zh - hi
1- Isolated iFrame Code (Ideal for Articles and Pages):
If you are publishing a guide, a deep‑dive article about pregnancy dating, or a resource for expectant parents — exactly like the one you are reading now — this method drops the fully functional calculator directly into the flow of your content. The isolated iframe keeps the tool contained, so your layout stays clean and your readers can calculate their due date, explore the four dating methods, and browse the week‑by‑week reference tables without reloading the page. Every feature works exactly as described: the four dating methods with their respective formulas, the cycle‑length and luteal‑phase adjustments, the coherent internal calendar that updates every date and milestone from a single reference point, the fertile window reconstruction, the trimester boundaries, and the full week‑by‑week size reference — all inside a small, self‑contained room on your site.
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2- Flexible Script (Suitable for Sidebars and Templates):
For sidebars, footers, or widget areas, this lighter integration loads the calculator on demand and adjusts to its container. It is the same engine, same four dating methods, same cycle‑length and luteal‑phase adjustments, same coherent internal calendar, same fertile window reconstruction, same trimester boundaries, same week‑by‑week reference tables, same six‑language support — just packaged for places where an iframe might feel heavy. Drop it into your template, and the calculator appears where you want it, when you want it. Your visitors can calculate their due date, track their gestational age, and explore the milestones without ever leaving your site.
<div class="tooliqo-tool" data-tool="pregnancy-calculator" data-lang="en"></div>
<script src="https://tools.tooliqo.co/embed.js" async></script>
Note: Both methods load the same calculator — the one whose four dating methods, cycle‑length and luteal‑phase adjustments, coherent internal calendar, fertile window reconstruction, trimester boundaries, week‑by‑week reference tables, and six languages you have just read about. The tool respects dark mode and reduced‑motion settings on your visitors' devices. No watermarks, no daily limits, no data collection. Just the calculator, doing what it does, on your site. And if a visitor enters their dates, sees their due date, and finally understands why their app and their midwife disagreed by five days — the tool has done what it was built to do. The rest is up to them, which was always the point.
What this guide covers
- What a pregnancy calculator actually calculates
- Gestational age vs. fetal age: the two-week gap that confuses everyone
- The four dating methods, and when to use each
- Every formula, laid bare
- Why 280 days? The strange history of Naegele's rule
- Ultrasound: the gold standard, and the rules for redating
- The fertile window and your estimated conception date
- Your prenatal timeline: fourteen dated milestones
- Trimesters, to the exact day
- Week-by-week size reference: full table
- Unit conversion formulas and quick values
- Preterm, term, post-term: the official definitions
- What are the odds you deliver on your due date?
- Irregular cycles, PCOS, twins and other special cases
- Eight mistakes people make with due date calculators
- How to use this calculator, step by step
- Frequently asked questions
- When to stop calculating and call your provider
1. What a pregnancy calculator actually calculates
A due date calculator does not predict your labour. It estimates a single reference point — the estimated due date (EDD), also written EDB (estimated date of birth) or EDC (estimated date of confinement, an older term) — and then derives every other number from it.
That reference point matters enormously, and not because of the birthday. Almost every clinical decision in pregnancy is indexed to gestational age. Whether a bleed at eleven weeks is investigated one way or another, whether a screening test is offered this week or next, whether steroids are given to mature a baby's lungs, whether labour is induced or allowed to continue, whether a newborn goes to the nursery or the neonatal unit — all of it hinges on the number of completed weeks. Get the dating wrong by ten days and you can shift a baby from "late preterm" to "early term" on paper, which changes the entire management plan.
So the real output of a good calculator is not one date. It is a coherent internal calendar:
- Estimated due date — day 280 of gestation
- Current gestational age — completed weeks plus days, written as 14+2 or "14 weeks 2 days"
- Days remaining — and, past 40 weeks, days overdue
- Trimester — with exact day boundaries, not vague thirds
- Estimated conception date — day 280 minus 266
- Fertile window — the six-day span in which conception most plausibly occurred
- Dated milestones — every screening window and appointment, converted from "weeks" into real calendar dates
The last item is the one people underestimate. "Book your anatomy scan between eighteen and twenty-two weeks" is an instruction you will forget. "Book your anatomy scan between 1 September and 29 September" is one you will act on. Converting gestational weeks into dates you can put in a phone calendar is quietly the most practical thing a pregnancy calculator does.
2. Gestational age vs. fetal age: the two-week gap that confuses everyone
Here is the single most common source of confusion in early pregnancy, and it is entirely the medical profession's fault.
Gestational age is counted from the first day of your last menstrual period (LMP). Fetal age — also called embryonic age, conceptional age, or post-ovulatory age — is counted from fertilisation. In a textbook 28-day cycle, ovulation happens on day 14, so the two numbers differ by about fourteen days.
Every clinical number you will ever be quoted uses gestational age. When your scan report says "12+3", it means twelve weeks and three days since your last period — the embryo itself has been developing for about ten weeks and three days. When you are told you are "four weeks pregnant" at a positive home test, the embryo is roughly two weeks old.
Why count from a period rather than from conception, which is obviously the real biological start? Because the LMP is a date you can actually report. Fertilisation is silent and invisible; almost nobody knows the day it happened. The onset of bleeding is unmistakable. Obstetrics chose the observable event over the meaningful one, and then built a 280-day convention around it, and here we all are.
The practical consequence: a pregnancy described as "40 weeks" involves about 38 weeks of embryonic and fetal development, plus two weeks of a cycle that preceded any embryo. Our calculator handles this transparently — when you enter a conception or ovulation date instead of an LMP, it internally derives the equivalent LMP reference (EDD minus 280 days) so that every downstream number stays on the standard clinical scale. Your gestational age will read correctly no matter which method you used to get there.
3. The four dating methods, and when to use each
Most online tools offer one input: last period. That is fine for a textbook cycle and quietly wrong for everyone else. Our calculator offers four methods, because four different pieces of information can be the most reliable thing you have.
Method 1 — Last menstrual period (LMP)
The default, and the right choice if your cycles are reasonably regular and you remember the date. Enter the first day of bleeding, not the last, and not the day you started spotting if the spotting was light and followed by a proper flow a day or two later.
Crucially, our tool also asks for your average cycle length (20–45 days) and luteal phase length (10–16 days). This matters more than most calculators admit. The classic formula assumes a 28-day cycle with ovulation on day 14. If your cycles run 35 days, you almost certainly ovulate around day 21, which pushes your true due date a full week later than the naive calculation. Ignoring cycle length is the single largest avoidable error in LMP-based dating.
Method 2 — Conception or ovulation date
Use this if you tracked ovulation with basal body temperature, cervical mucus, an ovulation predictor kit that turned positive, or a wearable that detected the LH surge or temperature shift. If you know the day you ovulated, you know your dating better than any LMP calculation can, because you have removed the follicular-phase variability entirely.
This method is also the honest choice for a single well-remembered act of intercourse in an otherwise unclear cycle — though be aware that sperm survive up to five days in the reproductive tract, so intercourse date and fertilisation date are not necessarily the same day.
Method 3 — IVF transfer or egg retrieval
IVF pregnancies are the most precisely datable pregnancies that exist, because the moment of fertilisation is known to the hour and recorded in a laboratory file. There is no estimation involved at all — only arithmetic. Our calculator supports three IVF anchors:
- Day-3 embryo transfer — the embryo is three days old on transfer day
- Day-5 blastocyst transfer — the embryo is five days old on transfer day (the most common modern protocol)
- Egg retrieval date — treated as day 0 of embryonic life
If you conceived through IVF, use this method and treat the result as definitive. Guidance from professional bodies is explicit that an IVF-derived due date should not be revised on the basis of a later ultrasound, because the ultrasound is the less precise measurement of the two. A frozen embryo transfer uses the transfer date with the embryo's age at freezing.
Method 4 — Ultrasound
Use this when you have a scan report in hand and it gives a gestational age. Enter the date the scan was performed and the gestational age it reported in weeks and days, and the calculator works backwards to the due date. This is the method to trust when your LMP is unknown, unreliable, or contradicted by the scan.
| Method | Typical margin of error | Best used when | Main weakness |
|---|---|---|---|
| IVF (known transfer or retrieval) | Effectively zero | Any assisted conception | Only available for IVF pregnancies |
| First-trimester ultrasound (CRL, 7–13+6 weeks) | About ±5 to ±7 days | Almost always, when available | Requires a scan; accuracy declines after 14 weeks |
| Known ovulation date | About ±1 to ±3 days | Cycle tracking, OPK, temperature charting | Depends on how well ovulation was detected |
| LMP with cycle adjustment | About ±1 to ±2 weeks | Regular cycles, confident LMP recall | Assumes consistent ovulation timing |
| LMP without cycle adjustment | Up to ±3 weeks with irregular cycles | Rough first estimate only | Systematically wrong for long or short cycles |
| Second- or third-trimester ultrasound | ±10 days to ±3 weeks, widening with gestation | Late presentation with no earlier data | Growth variation is mistaken for age variation |
Notice the pattern in that final row. Early in pregnancy, all embryos grow at nearly the same rate, so size is an excellent proxy for age. By the third trimester, genetics, placental function and maternal health have produced enormous size variation between babies of identical age — so a large baby looks "older" and a small baby looks "younger" when neither is. This is why an early scan beats a late one, and why a due date established in the first trimester is not revised later.
4. Every formula, laid bare
No black boxes. Here is exactly what the calculator computes.
| Method | Formula for estimated due date | Standard 28-day equivalent |
|---|---|---|
| Last menstrual period | LMP + (cycle length − luteal phase) + 266 days | LMP + 280 days |
| Conception or ovulation date | Conception + 266 days | Conception + 266 days |
| IVF, day-3 embryo transfer | Transfer date + 263 days | (266 − 3) |
| IVF, day-5 blastocyst transfer | Transfer date + 261 days | (266 − 5) |
| IVF, egg retrieval date | Retrieval date + 266 days | Retrieval = day 0 |
| Ultrasound | Scan date + (280 − gestational age in days at scan) | — |
Every other figure follows from the due date:
| Value | Formula |
|---|---|
| Dating reference (virtual LMP) | EDD − 280 days |
| Gestational age in days | Today − dating reference |
| Gestational age in weeks and days | ⌊days ÷ 7⌋ weeks, remainder days |
| Days remaining | EDD − today |
| Progress percentage | (gestational age in days ÷ 280) × 100 |
| Estimated conception date | EDD − 266 days |
| Estimated ovulation day (LMP method) | LMP + (cycle length − luteal phase) |
| Fertile window opens | Ovulation − 5 days |
| Fertile window closes | Ovulation + 1 day |
| Fetal (embryonic) age | Gestational age − 14 days |
Cycle length changes your due date. Here is by how much.
This table assumes a 14-day luteal phase, which is the population average. Read the last column as the correction the naive "LMP + 280" answer needs.
| Cycle length | Estimated ovulation day | Days from LMP to due date | Shift vs. the 280-day default |
|---|---|---|---|
| 21 days | Day 7 | 273 | 7 days earlier |
| 24 days | Day 10 | 276 | 4 days earlier |
| 26 days | Day 12 | 278 | 2 days earlier |
| 28 days | Day 14 | 280 | No shift |
| 30 days | Day 16 | 282 | 2 days later |
| 32 days | Day 18 | 284 | 4 days later |
| 35 days | Day 21 | 287 | 7 days later |
| 40 days | Day 26 | 292 | 12 days later |
| 45 days | Day 31 | 297 | 17 days later |
The luteal phase: the variable nobody asks about
Your cycle has two halves. The follicular phase runs from the first day of bleeding to ovulation, and its length varies a great deal — between individuals and between cycles in the same individual. The luteal phase runs from ovulation to the next period, and it is remarkably stable, typically 12 to 14 days, with a normal range of about 10 to 16.
This asymmetry is the key insight. When a cycle is long, it is almost always the follicular phase that stretched; the luteal phase stayed put. So the correct way to locate ovulation is to count backwards from the expected next period, not forwards from the last one. That is precisely what the formula cycle length − luteal phase does.
| Luteal phase | Estimated ovulation day | Days from LMP to due date |
|---|---|---|
| 10 days | Day 18 | 284 |
| 12 days | Day 16 | 282 |
| 14 days | Day 14 | 280 |
| 16 days | Day 12 | 278 |
If you have never measured your luteal phase, leave it at 14. If you have charted temperatures and know it runs short — say 11 days — entering that value gives you a genuinely better estimate than any calculator that does not ask.
A worked example
Suppose your last period began on 14 April, your cycles average 32 days, and your luteal phase is 13 days.
- Ovulation day = 32 − 13 = day 19 of the cycle
- Estimated ovulation and conception date = 14 April + 19 days = 3 May
- Estimated due date = 3 May + 266 days = 24 January the following year
- Dating reference (virtual LMP) = 24 January − 280 days = 19 April
- Fertile window = 28 April to 4 May
Note step 4. The naive answer would have been 14 April + 280 = 19 January. The cycle-adjusted answer is five days later, and the calculator shifts the entire internal calendar accordingly — so every milestone date, every screening window, and your current gestational age all move with it. That is what "coherent internal calendar" means in practice.
5. Why 280 days? The strange history of Naegele's rule
The 280-day convention is named after Franz Karl Naegele, a German obstetrician who published it in 1812. The rule as usually stated is charmingly analogue: take the first day of the last menstrual period, add one year, subtract three months, add seven days. Do the arithmetic and you land on 280 days later — nine calendar months and a week.
Naegele did not derive this from data in any modern sense. The likely origin traces further back, to the seventeenth-century Dutch physician Hermann Boerhaave, and it rests on two assumptions that were convenient rather than verified: that every woman has a 28-day cycle, and that conception occurs at a fixed interval after menstruation begins. Neither is true. The rule survived for two centuries because it is easy to do in your head and because it is, on average, close enough.
Modern data has repeatedly nudged at it:
- Mittendorf and colleagues (1990) found that among healthy, well-nourished, first-time mothers, the median duration of pregnancy ran closer to 288 days from LMP — eight days beyond Naegele's rule. Women who had given birth before averaged slightly shorter, around 283 days. This produced the alternative "Mittendorf–Williams rule," which is more accurate for specific populations but never displaced the simpler convention.
- Parikh's rule reworks the arithmetic to build cycle length in directly: add nine months to the LMP, subtract 21 days, then add your usual cycle length. For a 28-day cycle it reproduces 280 days exactly; for other cycle lengths it self-corrects. It is essentially the same logic our calculator uses, expressed in calendar months rather than days.
- Jukic and colleagues (2013) did something more interesting. Using precise hormonal markers to identify ovulation and implantation in naturally conceived pregnancies, they measured actual gestation length from conception rather than from a remembered period. The average was 268 days from ovulation to birth — and, strikingly, the range spanned 37 days among pregnancies that all ended in healthy term deliveries. Not 37 days of measurement error. Thirty-seven days of genuine biological variation in how long a normal human pregnancy lasts.
That last finding is the one to carry with you. Your due date is the midpoint of a wide, legitimate distribution. It is a statistical landmark, useful for organising care, and it is not a deadline you are failing to meet.
6. Ultrasound: the gold standard, and the rules for redating
Professional consensus is unambiguous: an ultrasound performed in the first trimester is the most accurate method available for establishing gestational age, and it should be used to confirm or revise LMP-based dating. The measurement that does the work is the crown–rump length (CRL) — the straight-line distance from the top of the head to the bottom of the torso, measured with the embryo in a neutral position.
CRL is reliable up to about 84 mm, which corresponds to roughly 13+6 weeks. Beyond that, sonographers switch to a composite of head circumference, biparietal diameter, abdominal circumference and femur length — and precision falls away, because by then babies of the same age genuinely differ in size.
Here is the part most people never see: there are formal thresholds governing when a scan overrides your LMP. If the discrepancy is smaller than the threshold, the LMP date stands. If it is larger, the pregnancy is redated to the scan.
| Gestational age at the time of the scan | Measurement used | Discrepancy that justifies redating |
|---|---|---|
| Up to 8+6 weeks | Crown–rump length | More than 5 days |
| 9+0 to 15+6 weeks | Crown–rump length | More than 7 days |
| 16+0 to 21+6 weeks | Biometry composite | More than 10 days |
| 22+0 to 27+6 weeks | Biometry composite | More than 14 days |
| 28+0 weeks and beyond | Biometry composite | More than 21 days |
Two practical implications follow. First, if your app and your scan differ by three days at nine weeks, nobody is going to change anything — and neither should you. Second, once a due date has been established and agreed, it should not keep drifting with every subsequent scan. Rediscussing dates at every appointment is how growth variation gets misread as dating error, and it makes late-pregnancy decisions harder rather than easier.
The exception, again, is IVF. A known transfer date outranks any ultrasound, at any gestation.
7. The fertile window and your estimated conception date
Our calculator reports a six-day fertile window and a most-likely conception date. Both are reconstructions, and understanding why they take the shape they do is genuinely useful.
An egg, once released, survives roughly 12 to 24 hours. Sperm, in the hospitable environment of fertile cervical mucus, can survive up to five days — occasionally a little more. Combine the two lifespans and you get a window that opens about five days before ovulation and closes about a day after it. That is why the calculator shows ovulation − 5 to ovulation + 1: six days in total, with the probability of conception rising steeply toward the end of that span and peaking on the day before and the day of ovulation.
This asymmetry surprises people. Intercourse four days before ovulation can produce a pregnancy; intercourse two days after ovulation essentially cannot. The window is front-loaded because sperm wait for the egg, never the reverse.
Two caveats on the conception date the calculator gives you:
- It is an estimate derived from your due date, not an observation. Unless you tracked ovulation, the date shown is a statistical best guess, typically accurate to within a few days.
- It is not a legal or forensic determination of paternity or timing. Given the six-day window and the variability of ovulation, no calculator can pinpoint a conception date precisely enough for that purpose. Only genetic testing can answer questions of parentage.
Ovulation itself is less punctual than diagrams suggest. Even among women with cycles they describe as regular, the day of ovulation varies substantially from cycle to cycle. Stress, illness, travel, poor sleep, hard training and significant weight change can all delay it. This is exactly why an early ultrasound is worth more than the best arithmetic — it measures what actually happened rather than what usually happens.
8. Your prenatal timeline: fourteen dated milestones
This is the timeline the calculator converts into real dates for you. Weeks are expressed as gestational age from the dating reference.
| Gestational age | Milestone | What it is and why it is scheduled then |
|---|---|---|
| 4+0 weeks | Home pregnancy test becomes reliable | Around the day of a missed period, urinary hCG has usually risen high enough for a home test to detect. Testing earlier produces false negatives, not false hope. |
| 8+0 weeks | First prenatal visit (window 8–10 weeks) | History, examination, baseline bloods, blood group and antibody screen, infectious disease screening, and confirmation of dates. Early enough for interventions to matter, late enough to be informative. |
| 10+0 weeks | NIPT / cell-free DNA screening becomes available | Enough fetal DNA is circulating in maternal blood to screen for common chromosomal conditions. It is a screening test, not diagnostic — a high-risk result leads to further testing, never to a conclusion. |
| 12+0 weeks | Dating and nuchal translucency scan (window 11+0–13+6) | The most accurate dating opportunity of the whole pregnancy, combined with measurement of the fluid space at the back of the neck as part of combined first-trimester screening. |
| 13+6 weeks | End of the first trimester | Miscarriage risk drops substantially, nausea often begins to settle, and the placenta has taken over hormonal support from the corpus luteum. |
| 20+0 weeks | Anatomy scan (window 18–22 weeks) | Systematic examination of fetal structures, placental position, amniotic fluid volume and growth. The organs are formed and large enough to see, while the baby is still small enough to image whole. |
| 24+0 weeks | Viability milestone | With modern neonatal intensive care, survival becomes a realistic prospect around this point, improving rapidly week by week. Exact thresholds differ between countries and hospitals. |
| 26+0 weeks | Glucose screening (window 24–28 weeks) | Placental hormones peak in their insulin-resistant effect during this period, so gestational diabetes reveals itself here — early enough that treatment still changes outcomes. |
| 27+0 weeks | Tdap vaccination window (27–36 weeks) | Vaccinating in this window maximises transfer of pertussis antibodies across the placenta, protecting the newborn during the months before their own vaccinations begin. |
| 28+0 weeks | Third trimester begins | Also the point at which anti-D prophylaxis is given to Rh-negative mothers, and when daily attention to fetal movement patterns becomes part of routine care. |
| 36+0 weeks | Group B Streptococcus swab (window 36+0–37+6) | Colonisation comes and goes, so the swab is timed close to delivery to be predictive. A positive result means antibiotics in labour, not a problem with the pregnancy. |
| 39+0 weeks | Full term begins | Neonatal outcomes are measurably best from this point. This is why elective deliveries without medical indication are generally not scheduled before it. |
| 40+0 weeks | Estimated due date | Day 280. The median, not the deadline. |
| 41+0 weeks | Late term begins | Monitoring typically intensifies and a conversation about induction usually happens, since risks begin to rise gradually beyond this point. |
Local protocols vary. Some health systems offer additional growth scans, different screening combinations, or routine appointments at intervals not listed here. Treat this table as a map of the standard landmarks, and your own care plan as the authority.
9. Trimesters, to the exact day
"Thirds of pregnancy" sounds simple until you try to say precisely when the second trimester starts. Here are the boundaries our calculator uses, which follow the most widely applied clinical convention.
| Trimester | Gestational age range | Days from dating reference | Duration |
|---|---|---|---|
| First | 0+0 to 13+6 | Days 0–97 | 98 days (14 weeks) |
| Second | 14+0 to 27+6 | Days 98–195 | 98 days (14 weeks) |
| Third | 28+0 to birth | Day 196 onward | 84 days to 40+0 |
The three parts are not equal, which is why our progress bar renders them at their true proportions — 98, 98 and 84 days — rather than as three identical thirds. A calculator that draws equal thirds is telling you a small lie about where you are.
Pregnancy "months" are messier still, because 40 weeks is not 9 calendar months. Here is the mapping in common use:
| Month | Weeks | Trimester |
|---|---|---|
| 1 | 1–4 | First |
| 2 | 5–8 | First |
| 3 | 9–13 | First |
| 4 | 14–17 | Second |
| 5 | 18–22 | Second |
| 6 | 23–27 | Second |
| 7 | 28–31 | Third |
| 8 | 32–35 | Third |
| 9 | 36–40 | Third |
Forty weeks is 280 days, which is nine calendar months plus about seven days — roughly 9.2 months. This is why "nine months pregnant" and "40 weeks pregnant" are not quite the same statement, and why clinicians stopped using months a long time ago.
10. Week-by-week size reference: full table
These are population averages drawn from standard fetal growth references, converted into both metric and imperial units. Two important notes on how to read them.
First, the length measurement changes basis at week 21. Up to week 20 it is crown–rump length — head to bottom, because the legs are curled and not usefully measurable. From week 21 it is crown–heel length, head to toe. That is why the number appears to jump by ten centimetres between weeks 20 and 21: the baby did not suddenly double, the ruler changed.
Second, and more importantly: these are averages, and individual healthy variation around them is wide. A baby measuring above or below the table is usually just a baby, not a problem. Growth is assessed by trajectory across multiple measurements and by percentile charts adjusted for maternal characteristics — never by comparison with a single average from a table on the internet.
| Week | Size comparison | Length (cm) | Length (in) | Weight (g) | Weight (imperial) |
|---|---|---|---|---|---|
| 4 | Poppy seed | ~0.1 | ~0.04 | <1 | — |
| 5 | Sesame seed | ~0.2 | ~0.08 | <1 | — |
| 6 | Lentil | ~0.4 | ~0.16 | <1 | — |
| 7 | Blueberry | ~1.0 | ~0.4 | <1 | — |
| 8 | Raspberry | 1.6 | 0.6 | 1 | 0.04 oz |
| 9 | Grape | 2.3 | 0.9 | 2 | 0.07 oz |
| 10 | Strawberry | 3.1 | 1.2 | 4 | 0.14 oz |
| 11 | Fig | 4.1 | 1.6 | 7 | 0.25 oz |
| 12 | Lime | 5.4 | 2.1 | 14 | 0.49 oz |
| 13 | Lemon | 7.4 | 2.9 | 23 | 0.81 oz |
| 14 | Peach | 8.7 | 3.4 | 43 | 1.5 oz |
| 15 | Apple | 10.1 | 4.0 | 70 | 2.5 oz |
| 16 | Avocado | 11.6 | 4.6 | 100 | 3.5 oz |
| 17 | Pear | 13.0 | 5.1 | 140 | 4.9 oz |
| 18 | Bell pepper | 14.2 | 5.6 | 190 | 6.7 oz |
| 19 | Mango | 15.3 | 6.0 | 240 | 8.5 oz |
| 20 | Banana | 16.4 | 6.5 | 300 | 10.6 oz |
| 21 | Carrot | 26.7 | 10.5 | 360 | 12.7 oz |
| 22 | Papaya | 27.8 | 10.9 | 430 | 15.2 oz |
| 23 | Grapefruit | 28.9 | 11.4 | 501 | 1 lb 1.7 oz |
| 24 | Ear of corn | 30.0 | 11.8 | 600 | 1 lb 5.2 oz |
| 25 | Cauliflower | 34.6 | 13.6 | 660 | 1 lb 7.3 oz |
| 26 | Lettuce | 35.6 | 14.0 | 760 | 1 lb 10.8 oz |
| 27 | Cabbage | 36.6 | 14.4 | 875 | 1 lb 14.9 oz |
| 28 | Eggplant | 37.6 | 14.8 | 1005 | 2 lb 3.5 oz |
| 29 | Squash | 38.6 | 15.2 | 1153 | 2 lb 8.7 oz |
| 30 | Cucumber | 39.9 | 15.7 | 1319 | 2 lb 14.5 oz |
| 31 | Coconut | 41.1 | 16.2 | 1502 | 3 lb 5.0 oz |
| 32 | Pineapple | 42.4 | 16.7 | 1702 | 3 lb 12.0 oz |
| 33 | Cantaloupe | 43.7 | 17.2 | 1918 | 4 lb 3.7 oz |
| 34 | Honeydew melon | 45.0 | 17.7 | 2146 | 4 lb 11.7 oz |
| 35 | Winter melon | 46.2 | 18.2 | 2383 | 5 lb 4.1 oz |
| 36 | Pumpkin | 47.4 | 18.7 | 2622 | 5 lb 12.5 oz |
| 37 | Watermelon | 48.6 | 19.1 | 2859 | 6 lb 4.9 oz |
| 38 | Watermelon | 49.8 | 19.6 | 3083 | 6 lb 12.8 oz |
| 39 | Watermelon | 50.7 | 20.0 | 3288 | 7 lb 4.0 oz |
| 40 | Watermelon | 51.2 | 20.2 | 3462 | 7 lb 10.1 oz |
| 41 | Watermelon | 51.7 | 20.4 | 3597 | 7 lb 14.9 oz |
| 42 | Watermelon | 51.5 | 20.3 | 3685 | 8 lb 2.0 oz |
Look at the weight column as a curve rather than a list. From week 8 to week 20, weight rises from one gram to three hundred — a three-hundred-fold increase, but in absolute terms almost nothing. From week 28 to week 40 it goes from about one kilogram to about three and a half. Well over half of a baby's birth weight is laid down in the final twelve weeks, most of it as fat and muscle rather than new structures. This is the physiological reason the third trimester feels the way it does, and the reason nutrition and rest in those final months are not merely comfort measures.
Development highlights by stage
| Weeks | Principal developments |
|---|---|
| 4–7 | Neural tube, heart and spinal cord form. The first heartbeat is usually detectable around week 6. Limb buds and facial structures begin. |
| 8–11 | Fingers, toes and eyelids take shape. The embryo is formally termed a fetus from week 9 and begins spontaneous movement, still too small to feel. |
| 12–15 | All major organs are present. The kidneys produce urine, contributing to amniotic fluid. Fine downy hair (lanugo) appears. Sex characteristics become distinguishable. |
| 16–19 | Hearing develops enough to register muffled sound. Many first-time parents feel initial fluttering movements (quickening) toward the end of this span; those who have been pregnant before often notice it earlier. |
| 20–23 | Sleep and wake cycles establish. Taste buds develop, skin thickens, and eyebrows and eyelashes form. |
| 24–27 | Lungs practise breathing movements and begin producing surfactant. Eyes open. The fetus reliably responds to familiar voices. |
| 28–31 | Rapid brain growth with the appearance of cortical folding. Bones harden while remaining flexible at the skull. Steady weight gain begins in earnest. |
| 32–35 | Most babies settle into a head-down position. Fat deposition smooths the skin. Immune antibodies transfer across the placenta in quantity. |
| 36–39 | The lungs mature toward independence. The baby often drops lower into the pelvis (lightening). Lanugo and vernix are largely shed. |
| 40+ | Fully developed. Labour may begin at any time. Continued growth is normal and expected in the final days. |
11. Unit conversion formulas and quick values
Every conversion the calculator performs, so you can check any figure by hand.
| Conversion | Formula | Worked example |
|---|---|---|
| Centimetres to inches | cm ÷ 2.54 | 16.4 cm ÷ 2.54 = 6.5 in |
| Inches to centimetres | in × 2.54 | 6.5 in × 2.54 = 16.5 cm |
| Grams to ounces | g ÷ 28.3495 | 300 g ÷ 28.3495 = 10.6 oz |
| Ounces to grams | oz × 28.3495 | 10 oz × 28.3495 = 283.5 g |
| Grams to pounds | g ÷ 453.592 | 3462 g ÷ 453.592 = 7.63 lb |
| Kilograms to pounds | kg × 2.20462 | 3.462 kg × 2.20462 = 7.63 lb |
| Decimal pounds to pounds and ounces | Whole part = lb; decimal × 16 = oz | 7.63 lb → 7 lb, 0.63 × 16 = 10.1 oz |
| Grams to kilograms | g ÷ 1000 | 1502 g = 1.502 kg |
| Conversion | Formula or value |
|---|---|
| Weeks to days | weeks × 7 |
| Days to weeks and days | ⌊days ÷ 7⌋ weeks, remainder = days |
| Weeks to calendar months | weeks ÷ 4.348 |
| Gestational age to fetal age | gestational days − 14 |
| Fetal age to gestational age | fetal days + 14 |
| Full term pregnancy | 40 weeks = 280 days ≈ 9 months 7 days ≈ 9.2 calendar months |
| Conception to birth (average) | 266 days from fertilisation |
| First trimester | 98 days |
| Second trimester | 98 days |
| Third trimester to 40 weeks | 84 days |
| Landmark | Gestational age | Days from dating reference |
|---|---|---|
| Home test reliable | 4+0 | 28 |
| First prenatal visit opens | 8+0 | 56 |
| NIPT available | 10+0 | 70 |
| NT scan window opens | 11+0 | 77 |
| NT scan window closes / T1 ends | 13+6 | 97 |
| Anatomy scan window | 18+0 – 22+0 | 126 – 154 |
| Viability milestone | 24+0 | 168 |
| Glucose screening window | 24+0 – 28+0 | 168 – 196 |
| Tdap window | 27+0 – 36+0 | 189 – 252 |
| Third trimester begins | 28+0 | 196 |
| GBS swab window | 36+0 – 37+6 | 252 – 265 |
| Full term begins | 39+0 | 273 |
| Estimated due date | 40+0 | 280 |
| Late term begins | 41+0 | 287 |
| Post-term begins | 42+0 | 294 |
12. Preterm, term, post-term: the official definitions
Until 2013, "term" covered everything from 37 to 42 weeks as though those five weeks were interchangeable. They are not. Outcomes at 37 weeks differ measurably from outcomes at 39. A joint reclassification introduced more precise categories, and these are now standard.
| Category | Gestational age at birth | Notes |
|---|---|---|
| Extremely preterm | Before 28+0 | Requires intensive neonatal care; outcomes improve rapidly with each additional week |
| Very preterm | 28+0 to 31+6 | Usually needs respiratory and feeding support |
| Moderate to late preterm | 32+0 to 36+6 | The largest preterm group; often needs some support with feeding, temperature or jaundice |
| Early term | 37+0 to 38+6 | Slightly higher rates of respiratory issues and feeding difficulty than full term |
| Full term | 39+0 to 40+6 | Optimal window for neonatal outcomes |
| Late term | 41+0 to 41+6 | Monitoring usually intensifies; induction is commonly discussed |
| Post-term | 42+0 and beyond | Risks to both mother and baby rise; most protocols recommend delivery before this point |
The practical takeaway from this table is that "full term" begins at 39 weeks, not 37. If you are 37 weeks and being told the baby is "term," that is true in the old sense and worth clarifying in the new one — particularly if a delivery date is being chosen rather than awaited.
13. What are the odds you deliver on your due date?
Low. Roughly one pregnancy in twenty — about five percent — ends on the exact estimated due date. Which is precisely what you would expect: if births are spread across a five-week window, no single day can hold a large share of them.
The more useful framing is the distribution rather than the point:
- The great majority of spontaneous births occur between 37 and 42 weeks
- A substantial proportion fall within roughly a week either side of the due date
- First pregnancies tend to run slightly longer than subsequent ones
- Twin and higher-order pregnancies deliver earlier as a rule, and are dated and monitored on a different schedule
- Length of gestation shows a familial pattern — if your mother and sisters carried long, you may too
Recall the Jukic finding: 37 days of natural variation among perfectly normal pregnancies. Your due date is drawn from the middle of that spread. The clinically meaningful question is never "am I past my date" but "is this pregnancy still progressing well" — which is why late-pregnancy care shifts from calendars to monitoring: fetal movement, growth, fluid volume, blood pressure and your own sense of how things are going.
There is an emotional dimension here worth naming. A date on a calendar becomes, in the mind, a promise. Passing it can feel like failure, and by 41 weeks the questions from everyone you know become genuinely wearing. It helps to reframe from the first day: your due date is the centre of a range, and a baby arriving at 39+2 or 41+3 is a baby arriving exactly on time.
14. Irregular cycles, PCOS, twins and other special cases
Irregular or unpredictable cycles
If your cycles vary by more than about a week from month to month, LMP-based dating is genuinely unreliable — not slightly imprecise, but potentially wrong by weeks. Use the calculator for an initial orientation, then prioritise an early ultrasound. If you have any ovulation tracking data, the conception-date method will serve you far better than the LMP method.
Polycystic ovary syndrome (PCOS)
PCOS frequently involves long cycles and unpredictable or absent ovulation, which makes the follicular phase both extended and variable. Our calculator accepts cycle lengths up to 45 days, which covers many PCOS cycles, but the underlying assumption — that ovulation occurs a fixed interval before the next period — holds less firmly when ovulation is irregular. Early ultrasound dating is particularly valuable here.
Conception soon after stopping hormonal contraception
The first few cycles after stopping the pill, the patch, the ring, an injection or an implant are often atypical in length. If you conceived in that window, your "average cycle length" is not yet a meaningful number. Enter your best estimate, note the uncertainty, and expect the scan to be the authority.
Conception while breastfeeding
Lactation suppresses and disorders ovulation, and cycles may not have resumed at all before conception. If you have had no periods, there is no LMP to enter — go straight to ultrasound dating, or use a conception date if you have one.
Bleeding in early pregnancy mistaken for a period
Implantation bleeding and other early-pregnancy spotting are sometimes recorded as a light period, which shifts the LMP forward by a full cycle and makes the pregnancy appear about four weeks younger than it is. If your dates and your scan disagree by roughly a month, this is a common explanation. The scan is right.
Twins, triplets and higher-order multiples
Multiple pregnancies are dated the same way as singletons — gestational age is gestational age — but their expected duration and care pathway differ substantially. Twins commonly deliver several weeks before 40 weeks, monitoring is more frequent, and the appropriate delivery timing depends on chorionicity, which is established by early ultrasound. Use the calculator for gestational age, and follow a multiples-specific care plan for everything else.
Assisted conception other than standard IVF
For intrauterine insemination (IUI), use the insemination date as the conception date — it is typically within a day of ovulation, which was usually triggered or monitored. For donor egg cycles, use the transfer date with the embryo's age at transfer, exactly as for standard IVF. For frozen embryo transfer, the transfer date and the embryo's age at freezing are what matter; the length of storage is irrelevant to dating.
15. Eight mistakes people make with due date calculators
- Entering the last day of the period instead of the first. This shifts everything by several days. The formula requires the first day of bleeding.
- Leaving cycle length at 28 when it is not 28. The most common and most consequential error. A 35-day cycle means a due date a full week later than the default calculation gives you.
- Confusing gestational age with fetal age. "Eight weeks pregnant" means eight weeks from your last period, not eight weeks of embryonic development.
- Using the conception method with an intercourse date. Sperm survive up to five days. The date of intercourse can precede fertilisation by most of a week.
- Recalculating after every scan. Once dating is established — ideally in the first trimester — it should stay fixed. Later scans assess growth, not age.
- Letting an ultrasound override an IVF date. The transfer date is more precise than any measurement. Keep it.
- Reading the size table as a target. Those are population averages. Healthy babies scatter widely around them, and a single measurement means very little without a trajectory.
- Treating the due date as a deadline. It is a median. Roughly a third of babies arrive after it, and that is entirely normal.
16. How to use this calculator, step by step
The calculator is designed to give a complete answer from a single date, then let you refine it.
- Choose your dating method. Last period, conception or ovulation, IVF transfer, or ultrasound. If you have more than one of these, use the more precise: IVF beats ultrasound, ultrasound beats ovulation, ovulation beats LMP.
- Enter the date. The calculator validates it — a future date, or one too far in the past to represent a current pregnancy, is rejected with an explanation rather than silently producing nonsense.
- Refine the secondary inputs. For the LMP method, set your average cycle length (20–45 days) and luteal phase (10–16 days). For IVF, choose day-3 embryo, day-5 blastocyst, or egg retrieval. For ultrasound, enter the gestational age the report gave, in weeks and days.
- Read the results. You get the due date in full written form, a progress ring, current gestational age, days remaining, current trimester, a proportional trimester bar, this week's size comparison with measurements, your fertile window and estimated conception date, the complete dated milestone timeline with completed items marked, and priorities for your current stage.
- Switch units if you prefer. Measurements toggle between centimetres and grams, and inches and ounces, with the toggle labelled in your own language.
- Copy your summary. One button copies your due date, gestational age, trimester and days remaining as plain text, ready to paste into a message, a note or an email to a family member.
Design details worth knowing
- Six languages. English, French, Spanish, Chinese, Hindi and Arabic, with full right-to-left layout for Arabic and units localised in each language rather than left in English abbreviations.
- Automatic language detection. The calculator reads the language of the page it appears on and matches it, with a manual selector if you want a different one.
- Nothing is stored. No account, no cookies, no local storage, no data leaving your device. Calculations happen in your browser and vanish when you close the page. Your dates are nobody's business but yours.
- Works on every screen. From a narrow phone in a waiting room to a desktop monitor, with a layout that adapts rather than merely shrinks. Dark mode is respected automatically, as is a reduced-motion preference.
- Accessible by design. Proper labels on every field, results announced to screen readers as they update, visible keyboard focus, and touch targets sized for real fingers.
17. Frequently asked questions
How many weeks pregnant am I right now?
Count the days from the first day of your last menstrual period to today, divide by seven, and you have completed weeks with the remainder as days. The calculator does this and adjusts for your cycle length, which the raw arithmetic does not.
Can I calculate my due date without knowing my last period?
Yes. Use the conception-date method if you tracked ovulation, the IVF method if you conceived through assisted reproduction, or the ultrasound method if you have a scan report with a gestational age on it. Any of these works without an LMP.
Why does my app give a different date from my midwife?
Almost always because one of you is using a cycle-adjusted calculation and the other is not, or because your due date has been revised following an ultrasound. A difference of a few days is normal and clinically irrelevant. Your provider's date is the one that governs your care, because it is the one written in your notes and used for every subsequent decision.
Which is more accurate, the calculator or the ultrasound?
The ultrasound, if it was done in the first trimester — unless you conceived through IVF, in which case the transfer date beats both. The calculator's job is to give you an immediate, well-reasoned estimate and to convert your dating into a usable calendar. It is not a substitute for a scan.
How early can a pregnancy test detect pregnancy?
Sensitive tests can sometimes detect hCG a few days before a missed period, but reliability improves substantially by the day the period was due, around 4+0 weeks. A negative test taken very early means "too early to tell," not "not pregnant." Retest in a few days.
What does 13+6 mean?
Thirteen completed weeks and six days of gestational age. The superscript notation is universal in obstetric records, so it is worth being fluent in it. It also means the very last day of the first trimester.
Is the fertile window shown in the calculator useful for trying to conceive?
The window shown here is retrospective — it reconstructs when conception probably occurred in a pregnancy that already exists. The same logic applies prospectively, though: the six days ending the day after ovulation are the fertile ones, with the highest probability on the day before and the day of ovulation. For actively trying to conceive, a dedicated ovulation tracker with real-time inputs will serve you better.
Can a due date change?
Yes, once — typically after a first-trimester ultrasound, if the discrepancy exceeds the thresholds in the redating table above. After that it should stay fixed for the rest of the pregnancy.
What if I go past 40 weeks?
You are in good company; a substantial share of pregnancies do. Between 40 and 41 weeks, monitoring generally increases. From 41 weeks, induction is usually discussed. Most protocols aim to deliver before 42 weeks, because risks rise gradually after that point. Nothing about passing your due date is a failure or an emergency.
Does the calculator work for twins?
The gestational age and due date it produces are correct for multiples, because dating does not change. What changes is the expected delivery timing and the care pathway, both of which are shorter and more intensive for twins. Follow a multiples-specific plan alongside the calculator's dates.
Are the fruit size comparisons real measurements?
They are intuitive analogies matched to the average length and weight for each week — a way to make a number like 16.4 cm feel like something. The actual measurements are in the table, and those come from standard growth references. Both are averages; both are approximate.
Is any of my data saved or sent anywhere?
No. Everything is computed in your browser. Nothing is stored, transmitted or logged, and closing the page erases it all.
18. When to stop calculating and call your provider
A calculator handles arithmetic. It cannot examine you, and it does not know your history. Contact your midwife, doctor or maternity unit promptly — do not wait for a scheduled appointment — if you experience any of the following:
- Vaginal bleeding at any stage of pregnancy
- Severe or persistent abdominal pain, or one-sided pain in early pregnancy
- A noticeable reduction or change in your baby's movement pattern in the second half of pregnancy
- A gush or steady trickle of fluid from the vagina
- Severe headache, visual disturbance, sudden swelling of the face or hands, or pain below the ribs
- Fever, or burning and urgency on passing urine
- Persistent vomiting that prevents you from keeping fluids down
- Regular painful contractions before 37 weeks
- Any strong sense that something is wrong, even without a symptom you can name — this is worth acting on, and clinicians would much rather see you unnecessarily than late
Important: this article and the calculator it accompanies are educational tools, not medical advice, diagnosis or treatment. Estimated due dates are statistical predictions derived from population averages. Individual pregnancies vary enormously, and only a qualified healthcare professional who knows your history can interpret what any of these numbers mean for you. Always follow the guidance of your own care team, and never delay seeking medical attention because of something you read online.
Key takeaways
- A due date is day 280 of gestation counted from your last menstrual period — a median, not a deadline. Around one pregnancy in twenty ends on the exact date.
- Gestational age is counted from your last period; fetal age from conception. They differ by about two weeks, and every clinical number uses the former.
- Cycle length matters. A 35-day cycle moves your due date a full week later than the default 280-day calculation, and most calculators never ask.
- The luteal phase is the stable half of your cycle, so ovulation is best located by counting backwards from the next expected period rather than forwards from the last one.
- First-trimester ultrasound is the most accurate dating method available, with formal thresholds governing when it should override an LMP-based date.
- IVF dating is effectively exact and should not be revised by a later scan.
- Full term now means 39+0 to 40+6 weeks, not 37 onwards — a distinction that matters if a delivery date is being chosen.
- Normal human pregnancy varies in length by more than five weeks. Your baby arriving at 39 weeks or 41 weeks is a baby arriving on time.
Use the calculator above for your dates, keep this page for the tables, and take both to your next appointment with better questions than you would otherwise have had.