How the due date is calculated
Pregnancy is counted from the first day of the last menstrual period, not from conception, so at conception you are already about two weeks pregnant on paper. Naegele's rule adds 280 days (40 weeks) to that date. For a 28 day cycle starting on 1 January 2026 this gives 8 October 2026.
The rule assumes ovulation on day 14. If your cycle is longer or shorter, ovulation usually moves by the same number of days, so the calculator shifts the due date: with a 32 day cycle it moves four days later. From a known conception date it adds 266 days (38 weeks). After an IVF transfer it adds 261 days for a day 5 embryo and 263 days for a day 3 embryo, because the embryo was already that many days old.
Weeks, trimesters and term
Weeks are written as completed weeks plus days: 12 weeks 3 days means you are in your 13th week. The first trimester runs to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth. Births from 37 weeks are early term, from 39 weeks full term and from 41 weeks late term; after 42 weeks a pregnancy is called post-term.
Only about one baby in twenty-five arrives on the due date itself. Most are born in the two weeks before or after it, which is why the calculator also shows the term window from 37 weeks to the end of week 41. An early ultrasound measurement is more precise than a date-based estimate, and your midwife or doctor may set a different due date after the dating scan.