Pregnancy Due Date Calculator

The formula used by the LMP method: due date = first day of last menstrual period + 280 days (40 weeks). It assumes a 28-day cycle with ovulation on day 14, and that gestation lasts 38 weeks from conception (so 40 weeks from LMP, because conception happens about 2 weeks into the cycle).

First-trimester ultrasound dating is the most accurate — it measures the embryo directly. ACOG recommends switching to the ultrasound-based due date if it differs from the LMP-based date by more than 7 days in the first trimester, or more than 10 days in the second trimester. If you know your exact conception date (e.g. IVF), that's also very accurate.

Adjust the cycle length input in the LMP method. The standard calculation assumes ovulation on day 14 of a 28-day cycle. If your cycle is 30 days, you ovulate on day 16, conception happens 2 days later, and your due date shifts 2 days later. If your cycle is 25 days, your due date is 3 days earlier.

Because the embryo is already partly grown at transfer. A day-3 transfer is a 3-day-old embryo; a day-5 transfer is a 5-day-old blastocyst. The due date is calculated as if the embryo is currently the equivalent gestational age. Day-3 transfer + 263 days = same as conception + 266 days. Day-5 transfer + 261 days = same as conception + 266 days.

12 weeks: end of first trimester, miscarriage risk drops sharply. 20 weeks: halfway point, detailed anatomy scan. 24 weeks: viability threshold (baby could survive with intensive care). 28 weeks: start of third trimester. 32 weeks: lung maturation. 37 weeks: full-term — birth from here is normal. 40 weeks: due date. 42 weeks: post-term — induction usually offered.

Trust the ultrasound, especially if done in the first trimester. ACOG guidelines: if first-trimester ultrasound disagrees with LMP by more than 7 days, the scan date becomes the official due date. Second-trimester ultrasound: 10-day threshold. Third-trimester ultrasound: 21-day threshold. Late-pregnancy scans aren't reliable for re-dating.