How the due date is calculated
Doctors use Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period, then adjust slightly for cycle length. If your cycle is longer or shorter than 28 days, the due date shifts accordingly — which is why this calculator lets you enter your cycle length.
Only about 4% of babies are born exactly on their due date, and roughly 60% arrive within a week of it on either side. The due date is a planning estimate, not a promise. If your periods are irregular or you are unsure of the date, an early ultrasound dating scan gives a more reliable estimate — ask your healthcare provider.
Pregnancy milestones
- Week 4-5: positive test is possible
- Week 8-12: dating ultrasound, first trimester screening
- Week 20: anatomy scan
- Week 40: estimated due date