Estimated due date by Naegele's rule: the first day of the last period plus 280 days.
By Naegele's rule: add 280 days — forty weeks — to the first day of the last period. The same result comes from subtracting three months and adding seven days and a year. The rule assumes a regular twenty-eight-day cycle.
Because the first day of the last period is known and the day of conception usually is not. Conception happens about two weeks later, so in the first two weeks of pregnancy there is no pregnancy yet. That offset is built into every clinical calculation.
It is an estimate, not a deadline: about five percent of births happen on the date itself, while ninety percent start within a two-week window around it. A first-trimester ultrasound gives a better date, and that is the one a doctor uses.
The first trimester runs to week thirteen, the second from fourteen to twenty-seven, the third from twenty-eight to birth. In Latvia maternity leave starts at week thirty, or at week twenty-eight for a multiple pregnancy.