Naegele's rule
The most common formula for an estimated due date (EDD) is Naegele's rule: EDD = LMP + 280 days. This assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the EDD shifts proportionally; an early ultrasound is the most reliable adjustment.
Why due dates are estimates
Only about 4% of babies arrive on their EDD. Anything between 37 and 42 weeks is considered a normal-length pregnancy. Spontaneous labour most often happens in the week before or the week after the EDD.
What happens at each milestone
- End of week 13 — first trimester ends; risk of miscarriage drops.
- Week 20 — half-way; anatomy ultrasound is typical.
- Week 28 — third trimester begins.
- Week 37 — early term; most babies do well from this point.
- Week 40 — estimated due date.
- Week 42 — post-term; most providers induce labour.
Not medical advice. This calculator is for general information. Talk to an obstetrician, midwife, or other qualified healthcare professional for any decision related to your pregnancy.