A pregnancy due date is one of the first numbers expectant parents receive, but the calculation behind it might be surprising: it's not counted from the actual date of conception, but from the first day of the last menstrual period — a full 280 days forward, based on a formula that's over a century old.
Naegele's Rule assumes a typical 28-day menstrual cycle with ovulation occurring around day 14. Since conception typically happens near ovulation, counting 280 days (40 weeks) from the last period lands roughly 266 days after actual conception — the extra two weeks account for the time before ovulation that's still counted as part of a "40-week" pregnancy by convention.
How the Due Date Is Calculated
Due Date = First Day of Last Menstrual Period + 280 Days
Current gestational age at any point is calculated as the number of days elapsed since the last period, divided by 7 to express it in weeks — which is why gestational age and actual embryonic/fetal age differ by about two weeks throughout a pregnancy.
A Worked Example
If the first day of the last period was March 1st, adding 280 days lands the estimated due date around December 5th. If today's date is May 15th (75 days after March 1st), current gestational age is 75 ÷ 7 ≈ 10 weeks, 5 days — even though actual embryonic age (from conception) would be closer to 8 weeks, 5 days.
Common Mistakes to Avoid
- Treating the due date as an exact prediction: only a small percentage of babies are actually born on their calculated due date — it's a statistical estimate, with a normal range of two weeks in either direction being common.
- Assuming everyone has a standard 28-day cycle: Naegele's Rule assumes this baseline — those with longer or shorter cycles may have their actual due date adjusted by a doctor using ultrasound dating instead.
- Confusing gestational age with fetal age: gestational age (counted from the last period) is about two weeks ahead of actual embryonic/fetal age (counted from conception) — both are used in different contexts.
- Not updating the estimate after an early ultrasound: ultrasound dating in the first trimester is often considered more accurate than last-period-based calculation alone, especially with irregular cycles.
Bottom Line
A due date is a well-reasoned estimate, not a guarantee. Use a Pregnancy Due Date Calculator to calculate your estimated due date and current gestational age from your last period, and confirm with your healthcare provider, especially if your cycle isn't a standard 28 days.