How Is Your Pregnancy Due Date Calculated?

When a doctor or an app gives you an estimated due date, it's almost always based on a simple rule that only needs one piece of information: the first day of your last menstrual period (LMP).

Naegele's Rule

Naegele's rule estimates a due date by adding 280 days (40 weeks) to the first day of your LMP. It assumes ovulation happens around day 14 of a 28-day cycle, and a full-term pregnancy lasts about 38 weeks from conception — which works out to 40 weeks from your LMP.

A Worked Example

If your LMP was 15 May 2024, adding 280 days lands on 19 February 2025 — that's your estimated due date.

Adjusting for Cycle Length

The 280-day rule assumes a 28-day cycle. If your cycles run longer or shorter, ovulation shifts too, so the due date shifts by the same amount: add the difference for a longer cycle, subtract it for a shorter one. A 32-day cycle, for example, pushes the due date about 4 days later than the plain 280-day estimate.

Gestational Age and Trimesters

Gestational age is counted from your LMP, not from the (later) date of actual conception — which is why you're already considered "2 weeks pregnant" before conception has technically happened. Trimesters follow the same clock: the 1st trimester runs through about week 14, the 2nd through week 27, and the 3rd from week 28 to birth.

Your due date is still an estimate — only about 5% of babies arrive on the exact date. Enter your LMP and cycle length into our pregnancy due date calculator below to see your due date, current gestational age, trimester, and days to go.