Due Date Calculator

Naegele's rule: 280 days from the first day of the last period. It assumes a 28-day cycle with ovulation on day 14, so a longer cycle pushes the date later and a shorter one pulls it earlier. This adjusts for the cycle length you enter.

Because the date of the last period is something people usually know, and the date of conception usually is not. Gestational age is counted from the last period by convention, which is why you are counted as two weeks pregnant at conception.

Only about 4% of births happen on the estimated date, and most fall within a fortnight either side. An ultrasound in the first trimester dates a pregnancy more accurately.

Then a date calculated this way is a weaker estimate, because it depends on ovulation being a predictable number of days after the period. A scan is the better answer.