How Pregnancy Due Dates Are Calculated (Naegele's Rule)
The 280-day rule behind due date estimates, and why only a small fraction of babies arrive exactly on that date.
A pregnancy due date is a statistical estimate derived from a simple day-count rule, not a precise prediction — and understanding the math clarifies why "due dates" are routinely off by more than a week in either direction.
Naegele's rule
Due Date = First Day of Last Menstrual Period (LMP) + 280 days (40 weeks).
Worked example
If 30 days have passed since the first day of the last menstrual period, the estimated due date is 280 − 30 = 250 days away. In weeks-pregnant terms, 30 days elapsed corresponds to 30/7 ≈ 4.3 weeks pregnant.
Why 280 days, specifically
Pregnancy is conventionally measured from the last menstrual period rather than from conception, since conception timing is rarely known precisely, while the start of the last period usually is. This convention adds roughly 2 weeks onto the actual ~266-day average gestation from conception, giving the standard 280-day (40-week) count from LMP.
Why the actual birth date varies so much
Only a small percentage of births occur exactly on the calculated due date — a range of about 3 weeks before to 2 weeks after is considered entirely normal (37-42 weeks gestation), which is why due dates are better understood as the center of a wide probability range than a precise prediction.
Why ultrasound dating can adjust the estimate
Early ultrasound measurements (particularly in the first trimester) can refine the due date estimate beyond Naegele's rule alone, especially for people with irregular menstrual cycles where the "first day of last period" assumption is less reliable. When the two methods disagree significantly, clinical guidelines typically favor the ultrasound-based estimate.
Common mistakes to avoid
- Treating the due date as a firm deadline rather than the midpoint of a normal, multi-week range
- Applying Naegele's rule without adjustment for irregular cycle lengths, which the basic 280-day formula assumes to be a standard 28-day cycle
- Confusing "weeks pregnant" (measured from LMP) with "weeks since conception" (roughly 2 weeks less) when comparing information from different sources
Estimate your own due date with the pregnancy due date calculator.