A due date is an estimate, not an appointment. It is the day you reach 40 weeks of pregnancy, counted in the way maternity care counts everywhere: from the first day of your last period, not from conception. Only about 4 in 100 babies arrive on the estimated date itself, and anywhere from 37 to 42 weeks is a normal length of pregnancy.
How it is calculated
- From your last period (Naegele's rule). Due date = first day of the last period + 280 days, which is 40 weeks. The rule assumes a 28-day cycle with ovulation on day 14, so this calculator shifts the date by the difference if your cycle is longer or shorter: a 32-day cycle adds four days.
- From conception or ovulation. Due date = that date + 266 days (38 weeks). Pregnancy is about two weeks shorter counted from conception than from the last period.
- From an IVF transfer. The embryo's age is known exactly, so the date is the most reliable of the three: transfer of a day-5 blastocyst + 261 days, transfer of a day-3 embryo + 263 days.
- Gestational age today is the number of days since the equivalent last-period date, written as weeks and days: 10 weeks 3 days is often shown as 10+3.
Example
Last period started 1 March with a 28-day cycle: due date 6 December, 280 days later. With a 32-day cycle the same start date gives 10 December. If instead you know you conceived on 15 March, the due date is 6 December as well, because 15 March is 14 days after 1 March.
Why a scan beats the calendar
If you are still counting cycles rather than weeks, the period calculator projects the next period and the ovulation calculator the fertile window. ACOG's position is that a first-trimester ultrasound, up to and including 13 weeks 6 days, is the most accurate way to establish gestational age. Ovulation does not always happen on day 14, and the remembered date of a last period is often approximate. If a scan date and a calendar date disagree by more than about a week early in pregnancy, the scan is normally used. When pregnancy came from IVF, the transfer date is used instead of either.
What the week numbers mean
The first trimester runs to 13 weeks 6 days, the second from 14 weeks to 27 weeks 6 days, the third from 28 weeks onwards. Babies born from 37 weeks 0 days to 38 weeks 6 days are early term, 39 weeks to 40 weeks 6 days is full term, 41 weeks is late term, and 42 weeks and beyond is post term.
Frequently asked questions
How is a due date calculated?
Add 280 days, or 40 weeks, to the first day of your last period. That is Naegele’s rule. If your cycle is not 28 days, the date shifts by the difference, which is what the calculator above does.
Why is pregnancy dated from my last period and not from conception?
The first day of bleeding is a date people can usually name, while the day of conception is rarely known. Counting from the last period adds about two weeks, which is why a pregnancy is 40 weeks from the period but 38 weeks from conception.
How accurate is a due date?
Only about 4% of babies are born on the estimated date. Roughly half arrive within a week either side, and 37 to 42 weeks is considered a normal length of pregnancy. A first-trimester scan narrows the estimate more than any calendar method.
How do I work out the due date after IVF?
Count from the transfer, because the embryo’s age is known: add 261 days for a day-5 blastocyst or 263 days for a day-3 embryo. Choose "IVF transfer" above and pick the embryo age.
Can the due date change?
Yes. If an early scan disagrees with the calendar date by more than about a week, your midwife or doctor will usually re-date the pregnancy from the scan and that becomes the date used for the rest of your care.
Sources
- ACOG Committee Opinion No. 700: Methods for Estimating the Due Date
- Committee Opinion No 700: Methods for Estimating the Due Date. Obstet Gynecol, 2017
- NHS: Pregnancy due date calculator — for the 37–42 week range
By Calcelate Team. Formula from the sources above.
- 2026-09-15 · Formula and texts checked