How Due Date Is Estimated: Naegele's Rule
The standard method for estimating a due date is Naegele's Rule, which adds 280 days (40 weeks) to the first day of your last menstrual period, assuming a typical 28-day cycle with ovulation around day 14:
Due Date = LMP + 280 days (adjusted for your actual average cycle length if different from 28 days)
Why This Is an Estimate, Not a Guarantee
Only about 5% of babies are born exactly on their estimated due date — most arrive within a 2-week window before or after. This calculation provides a useful planning estimate, but an ultrasound dating scan (especially in the first trimester) generally provides a more accurate estimate than LMP-based calculation alone, particularly for those with irregular cycles.
Pregnancy Stages by Week
- Weeks 1–12: First trimester
- Weeks 13–27: Second trimester
- Weeks 28–40+: Third trimester
Always confirm your due date and pregnancy progress with your healthcare provider, especially via ultrasound dating, for the most accurate assessment.