Pregnancy Due Date Calculator
Estimate your due date from last menstrual period, conception date or IVF transfer, with adjustment for cycle length. Includes gestational age today and the trimester milestone timeline.
Estimate your due date
Choose how you want to date the pregnancy, then enter that date. The result updates as you type.
Milestone timeline
How your due date is calculated
Almost every due date in the world starts from the same piece of arithmetic. It is called Naegele's rule, it was published by the German obstetrician Franz Karl Naegele in 1812, and it is disarmingly simple: add 280 days, or forty weeks, to the first day of your last menstrual period. That is the whole calculation. Two centuries of obstetric practice have added ultrasound, blood tests and fetal monitoring on top of it, but the number printed on the front of a maternity file still usually begins life here.
The oddity people notice first is that pregnancy is counted from the last period rather than from conception, which means the first two weeks of a forty-week pregnancy happen before there is anything to be pregnant with. That is not an error. It is a practical convention: the first day of bleeding is a date most people can name, while the day of ovulation usually passes unobserved. Dating from a knowable event and accepting a fixed offset is more reliable than dating from an event nobody witnessed.
Cycle adjustment = (your cycle length − 28) days
Worked example (last period began 1 March, 32-day cycle):
1 March + 280 + 4 = 10 December
The cycle adjustment matters because Naegele's rule quietly assumes a textbook 28-day cycle with ovulation on day 14. If your cycles run to 32 days, you most likely ovulate around day 18 rather than day 14, so conception happened four days later than the rule assumes and your due date moves four days later with it. Shorter cycles shift it the other way. This calculator applies that correction whenever you use the last-period method.
Conception and IVF dating
If you know the date of conception (from ovulation tracking, a single act of intercourse, or an insemination appointment), you can skip the assumption entirely. Ovulation is treated as day 14 of the notional cycle, so the offset becomes 280 minus 14, or 266 days from conception.
IVF is the most precisely dated pregnancy there is, because the age of the embryo at transfer is known to the day. A day-3 transfer places an embryo that is already three days past fertilisation, which is three days past the notional day-14 ovulation, so seventeen days of the forty weeks have already elapsed and 263 remain. A day-5 blastocyst transfer is two days further along again: nineteen days past the notional last period, leaving 261 days to the estimated due date. Those offsets are why an IVF due date should never be recalculated from a last menstrual period, which in a medicated cycle may not correspond to anything biologically meaningful at all.
| Method | Days added | Why that offset |
|---|---|---|
| Last menstrual period | 280 (± cycle adjustment) | Naegele's rule, corrected when your cycle is not 28 days |
| Conception or ovulation | 266 | 280 days minus the 14 days before ovulation |
| IVF day-3 transfer | 263 | The embryo is already 3 days past fertilisation at transfer |
| IVF day-5 transfer | 261 | A blastocyst is 5 days past fertilisation, so 19 days are already spent |
Source: American College of Obstetricians and Gynecologists, Committee Opinion No. 700, Methods for Estimating the Due Date, Obstet Gynecol 129:e150–e154 (2017). Naegele's rule itself dates to 1812 and has been in continuous clinical use since.
Why the estimate is a window, not a date
This is the most useful honest thing on the page, so it is worth saying plainly: only about four per cent of babies are born on their estimated due date. Roughly eighty per cent arrive somewhere inside the 37-to-42-week term window, and the remainder arrive earlier or later than that. A due date is best understood as the midpoint of a distribution, not as an appointment. The month is a good prediction. The day is not.
Part of the spread is simply biology. Labour begins when a particular pregnancy is ready, and readiness is not identical between people. But part of it is baked into the formula. Naegele's rule assumes a 28-day cycle with ovulation exactly on day 14, and that assumption is untrue for a very large share of people. Cycle length varies between individuals and from month to month within the same individual, and the timing of ovulation within a cycle varies more than the cycle length itself does. Every day of that variation is a day of error in the estimate before the pregnancy has even started.
This is why an early scan is worth more than any calculator. In the first trimester the embryo grows at a remarkably consistent rate between pregnancies, so measuring the crown-rump length (the distance from the top of the head to the bottom of the torso) dates the pregnancy to within about five days. That consistency fades as pregnancy progresses and individual growth patterns assert themselves, which is why a scan at thirty weeks dates a pregnancy far less precisely than one at nine weeks.
ACOG's position follows directly from that. When ultrasound dating and last-period dating disagree by more than a defined threshold, which is a matter of days in early pregnancy and widens as gestation advances, the ultrasound date is adopted and the pregnancy is redated. The last-period estimate is not discarded out of scepticism; it is simply the less precise of two measurements. If you have had a dating scan, the date it produced is the one to work from, and the figure this page gives you is a sanity check rather than a rival.
The trimester timeline
The milestones the calculator plots are the points where something changes clinically or where a routine appointment usually falls. Exact scheduling varies by country and by service, so treat these as the shape of a typical pregnancy rather than a booking diary.
| Week | Milestone | What it means clinically |
|---|---|---|
| 8–14 | Dating scan | Crown-rump-length measurement confirms gestational age and the number of embryos. This is the most accurate dating you will get. |
| 13 | First trimester ends | The period of organ formation is largely complete and the risk of miscarriage falls substantially from this point. |
| 14 | Second trimester begins | Often the most physically comfortable stretch of pregnancy; early nausea typically eases. |
| 18–22 | Anatomy scan | A detailed structural survey of the fetal brain, heart, spine, kidneys and limbs, plus the position of the placenta. |
| 24 | Viability considerations | The point around which neonatal intensive care can support survival outside the womb, though outcomes improve steeply with every further week. |
| 28 | Third trimester begins | Antenatal visits usually become more frequent; growth and fetal movement are monitored more closely. |
| 37–38+6 | Early term | Birth here is no longer premature, but outcomes are measurably better a week or two later. |
| 39–40+6 | Full term | The window with the best average outcomes for the newborn. Your estimated due date sits at 40+0. |
| 41–41+6 | Late term | Still normal. Monitoring usually increases and induction is commonly discussed towards the end of this week. |
| 42 onwards | Post-term | Rare, and actively managed, because the risks to the baby begin to rise beyond this point. |
Those last four rows were redefined in 2013. Until then "term" was a single undifferentiated block from 37 to 42 weeks, and a baby born at 37 weeks was described in exactly the same language as one born at 40. Evidence that outcomes continue improving through the final weeks led a workgroup convened by ACOG and the Society for Maternal-Fetal Medicine to split term into early, full, late and post-term. It is a small change in vocabulary with a real clinical point behind it: the last three weeks are not filler.
Where this estimate misleads
A calculator knows one date and one number. Everything else about your pregnancy is invisible to it, and there are specific, well-understood situations where that blindness matters.
Irregular or long cycles. The cycle-length adjustment on this page assumes your cycles are consistently that length. If they vary by a week or more from month to month (as they do in polycystic ovary syndrome, in the years approaching menopause, and in plenty of people with no diagnosis at all), then there is no single number to adjust by, and the estimate inherits all that variability. An early scan resolves this in a way arithmetic cannot.
Uncertain recall of the last period. Naegele's rule needs the first day of bleeding, not the day you noticed it or the middle of the week it happened in. Recall drifts, and light spotting in early pregnancy is easy to mistake for a period, which can pull the estimate a full month out of position. If you are not confident about the date, say so at your booking appointment rather than rounding to a guess.
Breastfeeding and recently stopped contraception. Cycles returning after hormonal contraception, or while breastfeeding, are often long, anovulatory or erratic for several months. The first period after either is a poor anchor for a formula built on regularity, and conception can occur before any period returns at all.
Twins, triplets and medical factors. A calculator cannot know that you are carrying more than one baby, that you have a condition such as pre-eclampsia or gestational diabetes, or that a previous birth was early. All of these shift expected delivery timing, and multiple pregnancies in particular are routinely planned to deliver well before forty weeks. Where your care team's expected date differs from this one, theirs is the number that reflects your circumstances.
What this calculator does and does not do
This tool applies a published dating rule to a date you supply. That is its entire scope. It tells you where a typical pregnancy of that gestational age would sit on a calendar, and it does so transparently, with the formula printed above so you can check the arithmetic yourself.
It does not assess the health of a pregnancy. It cannot tell you whether a pregnancy is progressing normally, whether the baby is growing well, or whether anything needs attention. No calculator can, because none of that information is contained in a date. It is also not a substitute for antenatal care. The scans, blood pressure checks, blood tests and conversations that make up a maternity pathway exist precisely because a calendar cannot see what they see.
If you have vaginal bleeding, abdominal pain, a reduction or change in fetal movement, severe or persistent headache, sudden swelling, or simply a feeling that something is not right, contact your midwife, obstetrician, maternity unit or emergency services. Do that at any hour and however minor it seems; maternity services expect these calls and would far rather hear from you early. A web page is a poor place to bring a worry, and it is not the right place to wait.