Naegele's rule
Due date = last period + 280 days (40 weeks), or last period + 7 days − 3 months + 1 year.
The rule assumes a 28-day cycle. A first-trimester ultrasound gives a more accurate estimate.
Frequently asked questions
Are babies always born on the due date?
No. Only about 5% arrive on the due date; normal birth happens between weeks 37 and 42.
Results are simulations to help planning and may differ from official calculations by agencies, employers or banks. Last updated 2026-11-02.