Due Date Calculator

The most common method (Naegele's rule) adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the date is adjusted accordingly.

An LMP-based calculator gives a useful estimate but only about 5% of babies are born on the exact date. Most healthy pregnancies deliver between 37 and 42 weeks. An early ultrasound (before 13 weeks) is the most accurate way to confirm the due date.

Cycle length affects when ovulation happens. The calculator above lets you set your cycle length (21–45 days). The due date shifts forward or backward by the difference between your cycle length and 28 days.

Yes. Use the Conception, Already-know-due-date, or Ultrasound options on this calculator. If none of these are available, an early dating ultrasound is the next-best option — your healthcare provider can do this in the first trimester.

By the time most women confirm pregnancy, the exact date of conception is unknown but the date of the last period is usually known. Counting from LMP gives a consistent reference point. It also means you are technically considered ‘two weeks pregnant’ before conception, which is an artefact of the convention.