Due Date Calculator

The standard rule counts 280 days, or 40 weeks, from the first day of your last menstrual period. ACOG describes an average pregnancy this way, and it is the same as Naegele's rule of adding 7 days, taking away 3 months and adding a year. A last period that started on 15 August 2026 gives a due date of Saturday, 22 May 2027. The count starts at the period, about two weeks before conception.

The 280-day rule assumes a 28-day cycle with ovulation around day 14. If your cycle is longer or shorter, the calculator moves the due date by your cycle length minus 28 days, the adjustment Perinatology.com and many clinicians use. A 35-day cycle moves that 22 May due date a week later, to 29 May 2027. If you know the day you conceived, the calculator adds 266 days instead.

IVF dates are the most exact, because the age of the embryo is known. ACOG Committee Opinion 700 says to date an IVF pregnancy from the transfer: 261 days after a day-5 embryo transfer and 263 days after a day-3 transfer. A day-6 blastocyst follows the same rule and adds 260 days. A day-5 transfer on 1 March 2026 gives 17 November 2026.

Most doctors confirm or set the due date with an early scan. ACOG says a first-trimester scan, measuring the baby's crown-rump length up to 13 weeks 6 days, is the most accurate way to date a pregnancy, to within about 5 to 7 days. Later scans are less exact: about 7 to 10 days between 14 and 21 weeks, and least reliable after 28 weeks. Choose Ultrasound and type the age the report gives, such as 8 weeks 3 days on 10 March 2026, and the calculator counts back to a due date of 17 October 2026.

When a scan and your period date disagree, the scan check uses ACOG's redating table. The guidance supports changing to the scan date only when the gap is larger than 5 days for a scan up to 8 weeks 6 days, 7 days from 9 weeks to 15 weeks 6 days, 10 days from 16 weeks to 21 weeks 6 days, 14 days from 22 weeks to 27 weeks 6 days, and 21 days from 28 weeks. The page shows both dates, the gap and which one the table supports.

The timeline turns weeks into real dates: the second trimester from 14 weeks, the third from 28 weeks, the WHO 2016 schedule of eight antenatal contacts (the first by 12 weeks, then 20, 26, 30, 34, 36, 38 and 40 weeks) and WHO's advice of one ultrasound before 24 weeks. It also marks ACOG's term labels: early term from 37 weeks 0 days, full term from 39 weeks 0 days to 40 weeks 6 days, late term from 41 weeks and postterm from 42 weeks. The calendar file adds each one as an all-day event.

Pregnancy dates are personal. Everything here is worked out in your browser tab: the dates you pick are not sent to TaskNinja, not saved and not linked to an account. We only count anonymous events, such as the calculator being used or the calendar being downloaded, never the dates themselves.

A due date is an estimate, not a promise. Full term alone covers two weeks, so treat the date as the middle of a window. Your doctor or midwife sets your official due date, usually from an early scan, and ACOG advises that once it is set it should change only in rare cases. Your own checkup plan may differ from the WHO schedule, especially in a high-risk pregnancy.