🤰 Pregnancy Due Date Calculator

Due dates — why the calculation works this way

The standard way to calculate a due date is Naegele's Rule: take the first day of your last menstrual period, add 9 months and 7 days (or equivalently, add 280 days). This has been the obstetric standard since the 19th century.

The slightly counterintuitive part: gestational age is counted from the first day of the last period, not from conception. Since ovulation typically occurs around day 14 of the cycle, a pregnancy described as "6 weeks along" is actually about 4 weeks from conception. This is why pregnancy is described as 40 weeks when the embryo/foetus is only about 38 weeks old.

• Accuracy: Naegele's Rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, your midwife or doctor may adjust the estimate.
• Dating scans: An ultrasound between 8–13 weeks gives a more accurate gestational age based on foetal size. If the scan date differs from Naegele's calculation by more than 5 days, the scan date is usually used.
• Overdue: A pregnancy is considered "post-term" at 42 weeks. Most hospitals will recommend induction between 41–42 weeks if labour hasn't started naturally.
• Only 4% on the due date: The due date is really the midpoint of a 5-week window (38–42 weeks). Most babies pick their own timing.