How a Due Date Is Calculated
A due date looks like a medical verdict, but it's a statistical estimate produced by a 200-year-old counting rule. Understanding how it's built explains its quirks — like why “40 weeks pregnant” includes two weeks before pregnancy began.
Naegele's rule: LMP + 280 days
The standard method adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). The historical shortcut — add 7 days, subtract 3 months, add a year — is the same arithmetic. LMP on January 1 gives a due date around October 8.
Why count from the period, not conception?
Because the LMP is a date most people actually know. Conception typically happens around ovulation, roughly two weeks after the period starts, and its exact date is usually unknowable. Medicine standardized on the observable marker, accepting the odd consequence that gestational age runs ~2 weeks ahead of embryonic age: at “8 weeks pregnant,” the embryo has existed for about 6.
If the conception date is known (as with IVF), you add 266 days instead — the same endpoint via the other starting line.
The cycle-length adjustment
Naegele's rule silently assumes a 28-day cycle with ovulation on day 14. Longer cycles usually mean later ovulation, so a good calculator shifts the due date by the difference: with a 31-day cycle, add 3 days; with a 26-day cycle, subtract 2. It's a small correction that meaningfully improves the estimate for the many people whose cycles aren't textbook.
How accurate is it?
Only about 1 in 20 babies arrives on the due date itself. The date is better read as the center of a window: most births happen between weeks 38 and 42, and clinicians treat that whole range as normal term. A first-trimester ultrasound, which sizes the embryo directly, dates a pregnancy more accurately than any period-based rule and will override the calculated date if they disagree by enough.
Weeks and trimesters
Convention slices the 40 weeks into trimesters at weeks 13 and 27. Week numbering confuses everyone at first: “12 weeks pregnant” means 12 completed weeks — you're in the 13th week. Appointment schedules, screening windows and “how far along” conversations all run on this completed-weeks convention.
Estimate yours — then confirm with a professional
The due date calculatorapplies Naegele's rule with the cycle-length correction (or the conception-date variant) and shows your current week, trimester and days remaining. It's a planning aid, not medical advice — your healthcare provider's dating, especially by ultrasound, is the one that counts. For general date arithmetic, the date difference calculator counts days between any two dates.