Pregnancy Due Date Calculator

How due dates are calculated — and why they are estimates

The most widely used method for calculating a pregnancy due date is Naegele’s Rule, named after the 19th-century German obstetrician Franz Karl Nägele. The rule is straightforward: add 280 days (40 weeks) to the first day of the last menstrual period. The logic is that pregnancy is conventionally measured from the LMP because that date is easier to know than the actual conception date, even though fertilisation does not occur until approximately two weeks after the period begins.

Why 40 weeks? A full-term pregnancy is defined as lasting between 37 and 42 weeks from the LMP. The 40-week standard (9 months and approximately one week) represents the average. In reality, only about 5% of babies are born on their calculated due date, and up to 80% of first-time mothers deliver after their due date. The expected delivery window — 37 to 42 weeks — spans five weeks, so the due date is best understood as the midpoint of a range rather than a precise prediction.

The LMP method assumes regularity. Naegele’s Rule presupposes a 28-day menstrual cycle with ovulation on day 14. Women with shorter cycles (e.g. 21 days) will ovulate earlier and may have a due date that is sooner than the formula suggests; those with longer cycles (e.g. 35 days) will ovulate later and may have a later true due date. If your cycle is irregular, an ultrasound-based calculation is more accurate.

First-trimester ultrasound is considered the gold standard for pregnancy dating. Between weeks 8 and 13, the foetal crown-rump length (CRL) varies little between babies of the same gestational age, making it an accurate dating tool. If the ultrasound date disagrees with the LMP date by more than 5 to 7 days, most obstetricians will update the due date based on the ultrasound. After 13 weeks, individual variation in foetal size increases, making ultrasound less reliable for dating.

IVF due date calculation is more precise because the exact date of fertilisation or embryo transfer is known. For a Day 5 blastocyst transfer, the due date is 261 days (37 weeks and 2 days) after the transfer date, which is equivalent to 266 days after fertilisation. For a Day 3 embryo transfer, add 263 days. This tool converts the IVF transfer date to an equivalent LMP date and then applies the standard 280-day formula for all downstream calculations.