Due date calculator

How a due date is worked out

The usual method is Naegele's rule: 280 days, that is 40 weeks, from the first day of the last period. That is what a midwife or a doctor starts from, which is why pregnancy is counted from a day when you were not yet pregnant.

The calculator also takes a cycle length. The 280-day rule assumes a 28-day cycle; a longer one means later ovulation and therefore a later due date. If you know the date of conception or an IVF transfer, count from that instead — those dates are exact and the estimate is tighter.

Along with the due date you get the current week, the trimester and the dates of the usual scans. Nothing is uploaded; the whole calculation runs in your browser, which matters for something this personal.

Frequently asked questions

How accurate is the due date?

It is an estimate. Only about four percent of babies are born exactly on it, and anything between weeks 37 and 42 counts as term. A date measured from an early ultrasound always takes precedence over a calculation.

Why does the cycle length matter?

Because ovulation happens roughly 14 days before the next period, not 14 days after the last one. With a 35-day cycle, conception falls about a week later than the standard rule assumes, and so does the due date.

How is an IVF transfer counted?

From the transfer date minus the age of the embryo: five days for a blastocyst, three for a day-three embryo. This is the most accurate starting point there is, because the date of fertilisation is known exactly.

Which week am I in?

Weeks of pregnancy are counted as completed weeks: at 12 weeks and 3 days you are in the thirteenth week. The tool shows both, so there is no confusion at an appointment.