Pregnancy due date calculator

Naegele’s rule adds 280 days to the last period. Only about 4% of babies arrive on the date it produces — it marks the middle of a range.

Why counting starts before conception

Gestational age is measured from the first day of the last menstrual period, not from conception. That adds roughly two weeks at the start during which no pregnancy exists. The convention persists because the date of the last period is something most people can report, while the date of conception usually is not.

So a pregnancy described as eight weeks along involves an embryo that has been developing for about six. Fetal age, used in embryology, counts from conception and runs two weeks behind the clinical figure.

The cycle length adjustment

Naegele's rule assumes a 28-day cycle with ovulation on day 14. Many cycles are longer or shorter, and the variation sits almost entirely in the first half — the luteal phase after ovulation is fairly consistent at around 14 days.

If your cycle averages 32 days, ovulation happens closer to day 18, so the due date shifts about four days later. The calculator applies this adjustment automatically when you enter a cycle length.

Ultrasound dating is more accurate

A crown-rump length measurement taken between 8 and 13 weeks predicts the due date more accurately than a reported last period, typically within about five days. Most providers will revise the estimated date if the scan differs from the LMP calculation by more than about a week.

Accuracy falls as pregnancy progresses, because growth becomes more variable between individuals. A scan at 30 weeks may be a week or more out either way, which is why early dating scans are preferred for establishing the date.

The due date is a midpoint, not a deadline

Only about one baby in twenty arrives on the estimated date. Roughly 80% are born between 37 and 42 weeks, and first pregnancies tend to run slightly longer than subsequent ones.

Term	Weeks
Preterm	Before 37
Early term	37–38⁺⁶
Full term	39–40⁺⁶
Late term	41–41⁺⁶
Post term	42 and beyond

These definitions matter clinically because outcomes differ modestly between early term and full term, which is part of why elective deliveries before 39 weeks are generally discouraged without a medical reason.

Common questions

How accurate is a due date from IVF?

More accurate than any other method, because the exact age of the embryo at transfer is known. A day-5 blastocyst transfer means the pregnancy is already 19 days along in gestational terms on the day of transfer, which removes the ovulation uncertainty entirely.

What if my periods are irregular?

The LMP method becomes unreliable, since it depends on ovulation happening at a predictable point. An early dating ultrasound is the better approach, and it is worth mentioning irregular cycles at your first appointment so dating is based on the scan.

Can the due date change during pregnancy?

Yes, usually only once and usually early. Providers generally revise the date if a first-trimester scan disagrees with the LMP estimate by more than five to seven days, then keep the revised date for the rest of the pregnancy even if later scans suggest something different.

What happens if I go past my due date?

Most providers monitor more closely after 40 weeks and discuss induction somewhere between 41 and 42 weeks, though practice varies by country and by individual circumstances. This is a conversation to have with your midwife or obstetrician rather than a decision a calculator can inform.