Due Date Calculator — LMP, IVF, ultrasound or conception

How due date is calculated

The standard estimate (Naegele's rule, 1812) adds 280 days to the first day of the last menstrual period (LMP), assuming a 28-day cycle and ovulation on day 14. From a known conception date, the same due date is 266 days out (280 minus the 14 days from LMP to ovulation).

Pregnancy is conventionally counted in 'gestational weeks' from LMP, not from conception, even though conception occurs about two weeks later. So 'six weeks pregnant' usually means six weeks since LMP, or about four weeks since conception.

Why your due date is an estimate

Only about 4% of babies arrive on the calculated due date. The 280-day formula assumes a 28-day cycle; real cycles range from 21 to 35 days, and ovulation timing varies even within a person from cycle to cycle. Most births fall in the two weeks before or after the calculated date — anywhere from week 38 to week 42.

First-trimester ultrasound dating (weeks 8-13) is more accurate than LMP-based dating, especially for irregular cycles. If you've had an early scan, your healthcare provider's recorded due date is more reliable than this calculator's number.

Trimester boundaries

First trimester: weeks 1-13 — embryo to fetus transition, organ formation. Second trimester: weeks 14-26 — fetal movement, anatomy scan, viability emerges around week 24. Third trimester: weeks 27-40+ — rapid growth and lung maturation. Many guidelines treat 37-42 weeks as 'full term', with births before 37 weeks classified as preterm.