Track pregnancy week by week with trimester dates and key milestones.
Gestational age is measured from the first day of the last menstrual period (LMP), not from conception, because LMP is more reliably known. A full-term pregnancy is 40 weeks (280 days) from LMP. The trimesters are weeks 1–12, 13–26 and 27–40. The due date from LMP is calculated using Naegele's rule: add 1 year, subtract 3 months, add 7 days.
Naegele's rule
EDD = LMP + 1 year − 3 months + 7 days (≡ LMP + 280 days)
This calculator is for informational and educational purposes only. It is not a substitute for prenatal care or advice from a qualified healthcare provider. Always confirm gestational age with your obstetrician or midwife.
The exact date of conception is usually unknown, whereas the start of the last period is a concrete, remembered date. Gestational age dating from LMP is universal in clinical practice and in this calculator.
First trimester: weeks 1–12 (conception through 12 weeks). Second trimester: weeks 13–26. Third trimester: weeks 27–40. These boundaries vary slightly by source; this calculator uses the most common clinical division.
Only about 5 % of babies are born exactly on their due date. Most births occur within 2 weeks before or after. An early ultrasound (before 13 weeks) is more accurate than LMP dating when cycles are irregular.