Pregnancy Due Date Calculator
How it works
The estimated due date is the first day of the last menstrual period plus 280 days — Naegele’s rule. Those 280 days assume ovulation on day 14 of a 28-day cycle, so the calculator adds the difference between your cycle length and 28 days on top: a 32-day cycle moves the estimate four days later, a 24-day cycle four days earlier. The estimated conception date moves with it, and is day 14 of the cycle by the same assumption.
Gestational age is counted from the first day of the last period rather than from conception, which is the convention pregnancies are dated in, and is why it runs about two weeks ahead of the age of the embryo. It is shown as weeks and days, so 12w 3d means twelve full weeks and three days. Measured this way the due date is always 40w 0d, whatever the cycle length.
All the arithmetic is whole days in UTC, so the result never shifts by a day because of a timezone. Counting days between two other dates? The date difference calculator does that.
Frequently asked questions
How is the estimated date worked out?
It adds 280 days — 40 weeks — to the first day of the last period, which is Naegele’s rule. The rule assumes ovulation on day 14 of a 28-day cycle, so a different cycle length is added on as a difference: a 35-day cycle moves the date seven days later.
How many births happen on the estimated date?
About 4% of them. The rest are spread over the weeks either side, which is why the result here is called an estimated date rather than a date. It is arithmetic on one date, not medical advice — a clinician dating a pregnancy uses scans and other information as well.
Where do the trimesters split?
This calculator starts the second trimester at 13 weeks 0 days and the third at 27 weeks 0 days, counted from the first day of the last period. Other sources draw both lines up to a week either side; there is no single standard.