Why Due Date Calculators Assume a 28-Day Cycle

Where the 280-day rule comes from

Naegele's rule, named after 19th-century German obstetrician Franz Karl Naegele, estimates a due date by adding 280 days (40 weeks) to the first day of the last menstrual period. That number assumes ovulation occurs roughly on day 14 of a standard 28-day cycle, with a full-term pregnancy lasting about 38 weeks from conception — which works out to 40 weeks measured from the last period instead, since conception typically happens about two weeks after a period starts.

Why cycle length actually matters

Someone with a 35-day cycle likely ovulates later than day 14, meaning conception — and the resulting due date — also lands later than the unadjusted rule would suggest. A calculator that accounts for average cycle length shifts the estimate accordingly, adding roughly one extra day for each day the cycle runs longer than 28 (and subtracting for shorter cycles), which produces a meaningfully more accurate estimate than blindly applying the 28-day assumption to everyone.

Why the due date is a range, not a deadline

Only around 1 in 20 pregnancies actually deliver on the exact calculated due date. A pregnancy reaching full term anywhere from three weeks before to two weeks after that date is entirely normal — the number is a statistical midpoint, not a prediction of the exact day, which is why clinical dating typically gets refined further with an early ultrasound rather than relying on the calendar method alone.

Try it yourself

Our Pregnancy Due Date Calculator applies Naegele's rule with a cycle-length adjustment, or calculates from a known conception date instead.