Pregnancy Due Date Calculator

Your estimated due date (EDD) marks the end of your 40-week pregnancy — or 280 days from the first day of your last menstrual period (LMP). This date is calculated using Naegele's Rule, developed by German obstetrician Franz Karl Naegele in 1812 and still used as the standard clinical method today.

Naegele's Rule: Add 1 year, subtract 3 months, and add 7 days to the first day of your last period. This works for cycles of 28 days. If your cycle is longer than 28 days, ovulation occurs later and the due date shifts accordingly; shorter cycles shift it earlier.

How accurate is the due date? Only about 5% of babies are born on their exact due date. About 80% are born within 2 weeks of the calculated due date. An early ultrasound (before 13 weeks) provides the most accurate dating — measurements of crown-rump length (CRL) can pinpoint gestational age to within 5–7 days. If your ultrasound date differs from your LMP date by more than 7 days, your provider will typically adjust the official due date to match the ultrasound.

Pregnancy timeline milestones: 8 weeks — heartbeat typically visible on ultrasound. 10–13 weeks — first trimester screening (NT scan, NIPT). 18–20 weeks — anatomy scan (detailed ultrasound). 24–28 weeks — glucose screening for gestational diabetes. 36–40 weeks — weekly appointments as delivery approaches.

Full term is 39–40 weeks. Early term is 37–38 weeks (baby is generally healthy but may need more support). Preterm is before 37 weeks. Post-term is after 42 weeks, at which point induction is typically recommended.

Always treat calculator results as estimates. Your OB, midwife, or nurse-midwife's clinical assessment — including ultrasound measurements — is the definitive source for your due date.