How due dates are estimated
Naegele’s rule: due date ≈ first day of the last menstrual period + 280 days (40 weeks). From conception, many charts add 266 days. Real pregnancies vary by one or two weeks. Ultrasound dating can replace this estimate.
Worked example
Naegele’s rule: last period 1 January 2026, due date about 8 October 2026 (add 1 year, subtract 3 months, add 7 days). A conception date is typically about 2 weeks later than LMP on a 28-day cycle.
Cycles are not all 28 days. Ultrasound dating can disagree with LMP. This is an estimate for planning, not medical advice. Confirm with a clinician.
Common questions
Why 40 weeks if conception is at week 2?
Gestational age counts from LMP, not from conception. That is why 40 weeks is the usual chart.
Irregular cycles?
This page assumes a 28-day cycle. Ask a clinician if your cycle is much longer or shorter.