How the due date is calculated
Doctors use Naegele's rule: the first day of the last period plus 280 days (40 weeks). For cycles longer or shorter than 28 days, the date shifts by the difference. For example, a period starting 1 January 2026 gives a due date of 8 October 2026.
Trimesters
- First: weeks 1–13
- Second: weeks 14–27
- Third: week 28 to birth
Few babies arrive exactly on their due date. Most are born within a couple of weeks either side. Your doctor may update the date after an early ultrasound.
For information only, not medical advice. Please follow your doctor's guidance.