The standard method (Naegele's rule) adds 280 days (40 weeks) to the first day of your last menstrual period, assuming a 28-day cycle. For longer or shorter cycles, the ovulation date is adjusted accordingly — ovulation typically occurs at cycle day (length − 14).
Only about 5% of babies are born on their exact due date. Most births occur within 2 weeks either side. NHS induction is typically offered at 41–42 weeks.
FAQs
A calculated EDD is an estimate — only about 4% of babies are born on their exact due date. An early ultrasound (8–13 weeks) gives a more accurate date as it measures actual fetal size. If your scan date and calculated date differ by more than 7 days in early pregnancy, the scan date is typically used.
For irregular cycles, an early dating scan (offered by the NHS between 10–14 weeks as part of the first trimester screening) is the most reliable way to establish your due date. The sonographer measures the crown-rump length of the embryo and calculates the gestational age from this measurement.
Most people choose to wait until after 12 weeks when miscarriage risk drops significantly and the dating scan has confirmed a healthy pregnancy. However, this is entirely a personal decision — some prefer to share early for support, others prefer to wait until after 20 weeks. There's no right answer.