How Is a Due Date Calculated?
The most widely used method for calculating a pregnancy due date is Naegele's rule, named after German obstetrician Franz Karl Naegele. The rule adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a standard 28-day cycle with ovulation occurring on day 14. For cycles that differ from 28 days, the due date is adjusted accordingly — longer cycles push the due date later, shorter cycles move it earlier.
It is important to understand that an estimated due date (EDD) is exactly that — an estimate. Only about 5% of babies are born on their exact due date. Most births occur within two weeks either side of the EDD. The due date is used as a reference point for monitoring fetal development and planning antenatal care, not as a precise prediction.
Pregnancy is divided into three trimesters. The first trimester (weeks 1–12) is the period of most rapid development, during which all major organs form. The second trimester (weeks 13–26) is typically the most comfortable period, with the baby growing rapidly and movements becoming noticeable. The third trimester (weeks 27–40) involves final development and preparation for birth.
Your midwife or doctor will typically confirm your due date with an ultrasound scan, usually performed between 10 and 14 weeks of pregnancy. The ultrasound measures the size of the baby and can provide a more accurate due date than LMP calculation alone, particularly if your cycles are irregular. Always consult your healthcare provider for personalised advice.