Pregnancy Due Date Calculator

How to use Due Date Calculator

1. 1
   Enter the first day of your last period. The first day of bleeding, not the last.
2. 2
   Set your usual cycle length. The default of 28 days is what the standard rule assumes.
3. 3
   Read the dates. Due date, trimester boundaries and the full-term window.

About calculating a pregnancy due date

A due date is calculated by Naegele's rule, which adds 280 days to the first day of the last menstrual period. The oddity that confuses people is that pregnancy is therefore dated from roughly two weeks before it began: conception typically occurs around day fourteen of the cycle, so a forty-week pregnancy involves about thirty-eight weeks of actual gestation. The convention exists because the date of a period is something a person knows and the date of conception is something almost nobody does. The rule's built-in assumption of a twenty-eight day cycle is where most of its error comes from. Cycle length varies substantially between people and somewhat within one, and the variation falls almost entirely in the phase before ovulation rather than after it. Someone with a thirty-two day cycle ovulates closer to day eighteen, meaning the pregnancy started four days later than the rule assumes, and the due date should move back by the same four days. What the calculation cannot do is predict a birth. Fewer than one in twenty babies arrive on the estimated date, and roughly eighty percent arrive within two weeks either side of it — which is why obstetric practice treats thirty-seven to forty-two weeks as the normal window rather than treating the due date as a deadline. A first-trimester dating scan is considerably more reliable, because embryonic growth rates are very consistent at that stage, and where a scan and a calculated date disagree by more than about a week the scan is what gets used.

Frequently asked questions