How the due date is estimated
The standard method, Naegele's rule, counts 280 days (40 weeks) from the first day of your last menstrual period. It assumes a 28-day cycle with ovulation around day 14, so if your cycle is longer or shorter this calculator shifts the date to match. Pregnancy is dated from the last period rather than conception because that date is far easier to know precisely.
Only about one baby in twenty arrives on the date
The due date is a midpoint, not a deadline. Fewer than 5% of babies are born exactly on it. A full-term birth is anything from 37 to 42 weeks, and first babies in particular often arrive a little later. Treat the date as the centre of a window a few weeks wide.
The three trimesters
The first trimester runs to the end of week 12, the second from week 13 to 26, and the third from week 27 to birth. The calculator shows which one you are in based on how many weeks have passed since your last period.
This is an estimate, not medical care
An early ultrasound, usually in the first trimester, dates a pregnancy more accurately than the last period alone, especially with irregular cycles, and your provider may adjust the date. This tool is for general information and planning. It does not replace antenatal care, and any concern about your pregnancy should go to a midwife or doctor. To count down the days to your due date, the date calculator helps.