Last updated 23 August 2026
How this is calculated
This uses Naegele's rule — the standard method of adding 280 days (40 weeks) to the first day of your last menstrual period. It assumes a typical 28-day cycle, which is why it's a starting estimate rather than a fixed date. An early ultrasound, if you have one, usually gives a more precise figure your doctor will use going forward.
Common questions
How is the due date calculated?
By adding 280 days (40 weeks) to the first day of your last menstrual period — known as Naegele's rule. It assumes a fairly typical 28-day cycle, which is why a doctor may adjust the estimate after an early ultrasound.
Why might my doctor give a different date?
Ultrasound measurements, especially in the first trimester, are often more precise than a date based on the last period, particularly if your cycle isn't exactly 28 days. Your doctor's estimate, once you have one, is the more reliable figure.
How accurate is a due date, generally?
Only about 1 in 20 babies are born on the exact estimated date. It's a useful planning midpoint — most births happen within about two weeks either side of it.