How a pregnancy due date is calculated

Steps

1. Note the first day of your last menstrual period. Here: 10 January 2026.
2. Add 280 days, which is 40 weeks: 17 October 2026.
3. If your cycle is not 28 days, add or subtract the difference. A 32-day cycle adds 4 days.
4. Enter the date and your usual cycle length in the due date calculator to see the due date.
5. Your doctor will confirm the date, often with an early scan.

How the cycle length changes the date

The rule assumes ovulation around day 14. With a 32-day cycle the same last period gives 21 October 2026, four days later. With a 26-day cycle it gives 15 October 2026, two days earlier.

The calculator also shows the likely conception date, about two weeks after the period began (24 January 2026 in the first example), and the start of the second trimester (18 April 2026) and the third (25 July 2026).

It is an estimate

Only about 1 in 20 babies arrives on the due date, and a healthy full-term birth can be a few weeks either side. An early ultrasound measures the baby and can correct the date if the last period was unsure or the cycle irregular.

Use it with your doctor

Use the date to plan visits, scans and leave, but let your doctor confirm it. Seek medical advice for any concern in pregnancy. This page gives general information, not medical advice.