Quick answer: A last period starting on 1 March 2026 with a 28-day cycle gives a due date of 6 December 2026: 280 days (40 weeks) later, by Naegele's rule.
How it's calculated
From the last period, the calculator uses Naegele's rule: due date = first day of the last menstrual period (LMP) + 280 days. With a cycle longer or shorter than 28 days, ovulation shifts, so the date moves by (cycle length − 28) days.
From conception, the due date is 266 days (38 weeks) after conception, because conception happens about two weeks after the LMP. For IVF the embryo's age is known exactly, so ACOG gives: due date = transfer date + 266 − embryo age, which is 263 days after a day-3 transfer and 261 days after a day-5 (blastocyst) transfer.
These are estimates from dates. A first-trimester ultrasound that measures the baby (crown–rump length) is the most accurate method, and your doctor will use it if it differs from the date by more than about a week. An IVF date is the exception: it is already exact and is not changed by a scan.
Worked examples
- LMP 1 March 2026, 28-day cycle: + 280 days = 6 December 2026
- Conception 15 March 2026: + 266 days = 6 December 2026
- Day-5 transfer on 10 April 2026: + 261 days = 27 December 2026
Frequently asked questions
How accurate is a due date?
It is an estimate. Birth anywhere from 37 to 42 weeks is common, and a due date is the middle of that range rather than a fixed day.
What if I don't know the date of my last period?
Use the conception date if you know it, or ask for a dating ultrasound. In the first trimester an ultrasound dates a pregnancy to within about a week.
Why is IVF dated differently?
The day of fertilisation and the embryo's age at transfer are known, so the due date is counted from the transfer: 266 days from fertilisation minus the days the embryo had already grown in the lab.
How many babies arrive on their due date?
Only about 1 in 20. Most babies are born within two weeks either side of it, so treat the date as the middle of a range.
Can my due date change?
Yes. If an early ultrasound differs enough from your period date, your doctor may set a new date. Scans in the first trimester are the most accurate for this.
What does full term mean?
Doctors call 39 weeks to 40 weeks 6 days full term, and 37 to 38 weeks 6 days early term. After 42 weeks is post-term.