Pregnancy Calculator

Pregnancy is conventionally dated from the last menstrual period (LMP) because that date is known precisely, while conception date is not. Conception typically occurs around day 14 of a 28-day cycle, so the 40-week pregnancy from LMP is actually about 38 weeks of fetal development. This convention has been used consistently in obstetrics for over 100 years.

Full term is defined as 39–40 weeks. Early term is 37–38 weeks; late term is 41 weeks; post-term is 42+ weeks. The American College of Obstetricians and Gynecologists (ACOG) discourages elective delivery before 39 weeks because the last 2–3 weeks of pregnancy include important brain and lung maturation.

If the first-trimester ultrasound (most accurate timing tool) differs from your LMP-based date by more than 7 days, your OB will likely use the ultrasound date. Update this calculator with the new due date and use 'Due Date' mode for more accurate tracking going forward.