Pregnancy Due Date & Fertile Window Calculator

How the due date is estimated

The standard method is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. It assumes a 28-day cycle with ovulation around day 14. If your cycle is longer or shorter, the date is usually shifted by the difference, because ovulation happens later in a longer cycle.

Pregnancy weeks are counted from the last period, not from conception, so at conception you are already about two weeks "pregnant" by this convention. That is why a full-term pregnancy is described as 40 weeks even though the baby develops for about 38.

Worked example

If your last period started on 1 March and your cycle is 28 days, adding 280 days gives 6 December. With a 32-day cycle, ovulation is likely four days later, so the estimate moves to about 10 December. For reference, the first trimester runs to the end of week 13, the second to week 27 and the third from week 28 until birth.

How accurate is it?

Only about 4 to 5 percent of babies arrive on the exact due date; most are born within two weeks either side. An early ultrasound scan (before about 14 weeks) measuring the baby's crown-rump length is more accurate than date-based estimates, and your doctor may update your due date based on it. Use this calculator for planning, and always follow your healthcare provider's dating.