Pregnancy Due Date Calculator

Naegele's rule, and the assumption inside it

The standard method adds 280 days to the first day of the last menstrual period. That builds in an assumption: a 28-day cycle with ovulation on day 14. For a 35-day cycle, ovulation happens roughly a week later and the due date shifts a week later with it. This page adjusts for cycle length; many quick calculators do not, which is a common source of a week's disagreement.

Why pregnancy is counted from before conception

Gestational age starts at the last period, roughly two weeks before conception actually occurs. It is a convention from an era before ovulation could be detected: the last period is something a person can report, while the moment of conception generally is not. So at “six weeks pregnant” the embryo is about four weeks old.

IVF dates are the precise ones

With IVF the fertilisation date is known exactly, so the estimate does not depend on cycle assumptions at all. A day-5 blastocyst transfer implies a due date 261 days later; a day-3 transfer, 263 days. These are the most accurate due dates available, and they are why IVF pregnancies are rarely re-dated by ultrasound.

The date is a midpoint, and ultrasound beats arithmetic

Only about 4 per cent of births happen on the estimated date, and roughly 80 per cent fall within two weeks either side. A first-trimester ultrasound measures the embryo directly and is more accurate than any date arithmetic; when the two disagree by more than about a week, clinicians generally re-date the pregnancy from the scan. Use this page for orientation, and your care team for decisions.

Frequently Asked Questions