Pregnancy due date calculator

The tool uses Naegele's rule: add 280 days, or 40 weeks, to the first day of your last menstrual period, which is the same as adding one year, subtracting three months and adding seven days. The rule assumes ovulation on day 14 of a 28-day cycle, so when your cycle is longer or shorter the tool shifts the due date by one day for each day of difference: a 35-day cycle adds a week.

Gestational age is counted from the first day of your last period, not from conception, because that date is usually known. Conception normally happens about two weeks later, so at the moment of conception you are already "2 weeks pregnant". The week count shown is weeks completed: 12 weeks 3 days means you are in your 13th week. The second trimester starts at 14 weeks 0 days and the third at 28 weeks 0 days.

It is an estimate: only around 4 in 100 babies arrive on the exact day, and most are born within a couple of weeks either side. A first-trimester ultrasound that measures the embryo is the most accurate way to date a pregnancy. If it disagrees with the period-based date by more than about 5 to 7 days, clinicians use the ultrasound date, so always treat the date your doctor or midwife sets as the official one.

With IVF the age of the embryo is known exactly, so the due date is 266 days after fertilisation. For a day-3 transfer the embryo is already three days old, which gives transfer date plus 263 days; for a day-5 blastocyst transfer it is transfer date plus 261 days. Cycle length is ignored for IVF and conception dating because ovulation timing is not being guessed.

Babies born from 37 weeks 0 days to 38 weeks 6 days are early term, 39 weeks 0 days to 40 weeks 6 days is full term, 41 weeks 0 days to 41 weeks 6 days is late term, and 42 weeks 0 days or later is post-term. The term window shown runs from 37 weeks to 42 weeks; most doctors discuss induction when a pregnancy approaches 41 to 42 weeks.