Pregnancy Due Date Calculator

How is the due date worked out?

By Naegele's rule: 280 days — 40 weeks — added to the first day of your last menstrual period. That is the standard convention used to date a pregnancy from an LMP, and it is the entire calculation this tool performs.

Why is it dated from my last period rather than conception?

Because the first day of a period is a date most people can identify, while the day of conception usually is not. It is a practical anchor, not a biological one — which is why the count starts roughly two weeks before conception could have occurred. That is also why "6 weeks pregnant" means about four weeks since conception.

What if my cycle is not 28 days?

Then the estimate is likely off. Naegele's rule assumes a 28-day cycle with ovulation around day 14. If your cycles run consistently longer or shorter, ovulation shifts with them and the true due date shifts by roughly the same number of days. This tool applies the standard 280-day rule without adjusting for cycle length, so tell your clinician your usual cycle length — they can correct for it.

Why did my clinic give me a different date after the scan?

Ultrasound dating measures the embryo or fetus directly instead of inferring from a calendar, so it does not depend on knowing your LMP or your ovulation day. When an early scan disagrees with the LMP estimate by more than a set margin, clinicians generally re-date the pregnancy from the scan. The scan date is the one to trust.

What do the week and day counts mean?

Gestational age, expressed the way clinics write it: completed weeks plus extra days since your LMP. "12 weeks and 3 days" means 87 days have elapsed. The tool also shows which trimester that falls in — weeks 0 to 12 are the first, 13 to 26 the second, and 27 onwards the third — and how many days remain until the 280-day mark.

Will the baby arrive on this date?

Almost certainly not. A due date is the midpoint of a range, not an appointment; full term spans several weeks around it, and first pregnancies commonly run past the estimate. Treat it as the centre of a window your clinician will manage.

Does this work if I conceived through IVF?

Not reliably. IVF dating starts from a known transfer or retrieval date and uses a different offset depending on the embryo's age at transfer, so an LMP-based rule does not apply cleanly. Use the date your fertility clinic gave you.

Is anything I enter stored?

No. The date arithmetic happens in your browser and nothing is transmitted or saved.

Can I rely on this instead of seeing a clinician?

No. This is a calendar calculation, not prenatal care. Dating, screening and every decision that follows belong with a midwife or doctor — book that appointment regardless of what this page says.