👶 Pregnancy Due Date Calculator

How a due date is estimated

The standard method, known as Naegele's rule, adds 280 days — 40 weeks — to the first day of your last menstrual period. This calculator applies that rule.

The estimate assumes a regular 28-day cycle with ovulation on day 14. If your cycles are consistently longer or shorter, the date shifts accordingly: a 32-day cycle typically moves the due date about four days later.

Pregnancy is conventionally dated from the last period rather than conception, which means you are counted as two weeks pregnant at the moment of conception. It is a confusing convention, but it is used because the date of the last period is knowable while the date of conception usually is not.

The due date is a midpoint, not a deadline

Only around one in twenty babies arrives on the estimated date. Roughly 80 percent of births occur within the two weeks either side, and the distribution is wide enough that treating the date as fixed causes unnecessary anxiety.

Full term is defined as 39 weeks 0 days to 40 weeks 6 days. Deliveries from 37 weeks are early term, 41 weeks is late term, and beyond 42 weeks is post-term. First pregnancies tend to run slightly longer than subsequent ones.

An early ultrasound — typically between 8 and 13 weeks — measures the embryo directly and is more accurate than the last-period calculation. Where the two disagree by more than about a week, clinicians usually adopt the scan date.

The three trimesters

Viability outside the womb improves substantially from around 24 weeks with modern neonatal care, though outcomes at that stage remain highly variable.

Important note

This calculator provides a general estimate only and is not medical advice. Pregnancy dating, monitoring, and any decisions about care should be made with a qualified healthcare provider who can assess your individual circumstances. If you have irregular cycles, are unsure of your dates, or have any concerns, speak to a midwife or doctor.

Common questions

What if I do not know my last period date?

An early ultrasound can date the pregnancy without it, and is the standard approach in that situation.

Does IVF dating work differently?

Yes. With IVF the transfer date is known precisely, so the due date is calculated from it and is considerably more accurate than the last-period method.

Can the due date change?

It can be revised after an early scan. Later scans are generally not used to change dates, because growth variation between individual babies increases as pregnancy progresses.