How Due Dates Are Calculated

In the early 1800s, German obstetrician Franz Naegele formalised the rule still used today: take the first day of your last menstrual period (LMP), add one year, subtract three months, then add seven days. Worked through, that is simply your LMP plus 280 days — 40 weeks exactly.

Why count from a period that happened before you were even pregnant? Because the LMP is a date most women can pinpoint, while conception usually cannot be. The rule assumes a 28-day cycle with ovulation — and therefore conception — on day 14. Conception to birth is about 266 days (38 weeks), and 266 + 14 = 280.

If your cycle is 32 days, you probably ovulate around day 18, not day 14 — so a fair estimate adds those extra 4 days to the due date. A 24-day cycle subtracts 4. Our due date calculator applies this adjustment automatically when you enter your cycle length. Note the adjustment only helps if your cycles are regular; with irregular cycles, no calendar formula can be precise.

In the first trimester, embryos grow at a remarkably consistent rate, so measuring the crown–rump length on a scan before about 13 weeks can date a pregnancy to within roughly 5–7 days. That is why, if your scan estimate differs from your LMP estimate by more than a few days early on, most providers adopt the scan date. Later scans are less reliable for dating because babies start growing at individual rates — a big baby is not necessarily an older baby.

With IVF the key dates are known to the day, so dating is exact: due date = fertilisation date + 266 days. Clinics translate embryo transfers into the same framework — a day-3 embryo transfer counts as day 17 of a cycle, and a day-5 blastocyst transfer as day 19 — then count 280 days from the theoretical LMP.

Only around 1 in 20 babies (about 5%) are born on their estimated due date. Around 7 in 10 arrive within ten days either side of it. A pregnancy is considered full term between 39 and 40 weeks 6 days, and it is normal to deliver anywhere from 37 to 42 weeks. The due date's real job is scheduling: it times your scans, screening tests and growth checks, and flags if a pregnancy runs long — it was never a promise about the day itself.

Ready to find yours? Try the due date calculator or, if you are still trying to conceive, the ovulation calculator.