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Due Date Calculator

Pregnancy due date calculator: the due date from your last period and cycle length, conception date, IVF transfer or ultrasound, and how many weeks pregnant.

Choose how the pregnancy is being dated — last period, conception, IVF transfer or an ultrasound — and enter that date. A first-trimester scan is the most accurate and takes precedence over the calendar.

How the pregnancy is dated

The last period is the usual starting point. Conception is exact if you know it. IVF is the most precise of all. An ultrasound in the first trimester is more accurate than any of them and is what a clinic will use.

The date for the method above: the first day of your last period, the conception date, the transfer date, or the date of the scan.

First day of one period to the first day of the next. 28 is the textbook figure and few people match it exactly. A longer cycle ovulates later, so the due date moves later by the same number of days.

A day-5 blastocyst transfer gives a due date 261 days later; a day-3 transfer 263 days. The days the embryo spent in culture are already part of its age, which is why the two differ.

The gestational age the scan measured, in whole weeks. Crown–rump length in the first trimester dates a pregnancy to within about 5 to 7 days.

The days part of the measurement: a scan reading "8 weeks 3 days" has 3 here.

The date to report gestational age and the trimester as of. Defaults to today.

Results

Assumptions and formula

Pregnancy is dated as 280 days from the first day of the last menstrual period, or from a notional one worked back from whichever anchor you gave: conception less 14 days, an IVF transfer by ACOG’s offsets (261 days for a day-5 embryo, 263 for a day-3), or the scan date less the gestational age it measured. Gestational age, the trimester and the milestone dates are then counted from that dating start.

A cycle that is not 28 days moves the last-period estimate by the difference, because ovulation is taken to happen 14 days before the next period rather than on day 14. That adjustment is a standard convention rather than part of ACOG’s guidance, and it only helps when cycles are regular. Where an early ultrasound differs from the last-period date by more than the margin ACOG sets for that stage — 5 days before 9 weeks, 7 days from 9 to under 14 — the ultrasound date is the one used clinically.

Worked example

A last period starting 1 January 2026 on a 28-day cycle gives a due date of 8 October 2026. On a 35-day cycle the same date gives 15 October, a week later. A day-5 embryo transferred on 20 January 2026 also gives 8 October. A scan on 12 March 2026 measuring 8 weeks 3 days dates the pregnancy from 12 January and gives 19 October.

Frequently asked questions

How is a due date calculated from the last period?

By adding 280 days — 40 weeks — to the first day of the last menstrual period. This is Naegele’s rule and it assumes a 28-day cycle with ovulation on day 14, so conception is actually about two weeks after the date the count starts from. That is why a pregnancy is called "4 weeks" at the point of a missed period, when the embryo is two weeks old.

Does my cycle length change the due date?

By the difference from 28 days, if your cycles are regular. Ovulation tends to happen about 14 days before the next period, so a 35-day cycle ovulates around day 21 and the due date is 7 days later than the plain rule gives. Enter your usual cycle length and the estimate shifts accordingly. Irregular cycles make the last-period method unreliable, which is one reason an early ultrasound is used to confirm dating.

Which is more accurate, the last period or the ultrasound?

A first-trimester ultrasound, measuring crown–rump length, is the most accurate method — within about 5 to 7 days. ACOG’s guidance is that when an early ultrasound differs from the last-period date by more than a set margin (5 days before 9 weeks, 7 days from 9 to under 14 weeks), the ultrasound date is used. Later scans are less precise, so a due date set early is normally kept even if a later scan measures differently.

How is the due date worked out after IVF?

From the transfer date and the embryo’s age. ACOG gives 261 days after a day-5 (blastocyst) transfer and 263 days after a day-3 transfer, because the days the embryo spent in culture are already part of its age. Egg retrieval date plus 266 gives the same answer. IVF dating is the most precise of all, since conception is known to the day.

What are the chances of giving birth on the due date?

Around 4%. In a large study of naturally conceived pregnancies with known ovulation dates, births spread over roughly five weeks, with the median a few days after 40 weeks. Term is now described as a range — early term from 37 weeks, full term from 39, late term at 41, post-term from 42 — and the due date is the middle of it, not a deadline.

How many weeks pregnant am I, and does the method change the answer?

Count from the first day of your last period, not from conception: a pregnancy week calculator puts you at week 6 two weeks after a missed period, because the clock starts about two weeks before conception. A conception calculator or conception date calculator working from a known date therefore adds 14 days to reach the same dating. An IVF due date calculator is the most exact of all, because the transfer date and the embryo age are known to the day. Whichever anchor you enter, this page reports the weeks and days today and the same due date.

Sources

Sources reviewed 13 September 2026: checked against their current editions on that date.

This page is an educational estimate and is not medical advice. Follow individual guidance from your clinician.

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