The math
The standard pregnancy due date - what doctors call the "EDD" or estimated date of delivery - is calculated by Naegele's rule: add 280 days to the first day of the last menstrual period.
EDD = LMP + 280 days
or: EDD = LMP + 9 months + 7 days
The 280-day number assumes a 28-day cycle with ovulation at day 14. Conception then happens around day 14, and pregnancy is dated from the LMP - so "40 weeks of pregnancy" actually includes about 2 weeks before conception. For longer or shorter cycles, the calculator adjusts the conception date accordingly.
Frequently asked
Why does pregnancy start before conception?
For historical and practical reasons. The LMP is a date most people can remember; the actual conception date usually can't be pinpointed. So the medical convention dates pregnancy from the last period, even though the embryo doesn't exist until about two weeks in.
How does ultrasound dating work?
Early ultrasounds (before 14 weeks) measure the embryo or fetus's size - particularly the crown-rump length - and compare it to growth charts. Early in pregnancy, growth is highly predictable across individuals, which makes it the most accurate dating method. Later in pregnancy, individual variation makes ultrasound less precise.
Should I be worried if I'm past my due date?
About 5% of babies are born on their estimated due date. 50% are born within a week on either side. Most providers will discuss induction options between 41 and 42 weeks if you haven't delivered. Going to 41 weeks is normal; going past 42 is when most providers intervene.