Calculate your estimated due date using last menstrual period or conception date, and track your pregnancy timeline.
From LMP, EDD is calculated by adding 280 days (Naegele's rule). From conception date, EDD is conception + 266 days (38 weeks). Current gestational week counts from LMP.
Naegele's rule is a standard method for calculating estimated due dates, developed by German obstetrician Franz Karl Naegele in the early 1800s. It works by adding 280 days (40 weeks) to the first day of the last menstrual period. The formula assumes a regular 28-day cycle and ovulation on day 14. Many modern dating methods use early ultrasound instead, which can be more accurate when cycle length varies. Naegele's rule remains widely used as a baseline estimate.
Only about 5% of babies are born on their calculated due date. The vast majority arrive within a 2-week window on either side — between 38 and 42 weeks of gestation is considered normal. Early ultrasound dating (before 14 weeks) tends to be more accurate than LMP-based calculations because it directly measures fetal size. Factors like irregular cycles, unknown LMP, and natural variation in fetal development all contribute to uncertainty. A due date is best understood as the midpoint of a likely delivery window.
Gestational age counts from the first day of the last menstrual period (LMP), which is typically 2 weeks before fertilisation actually occurs. This is the standard used in obstetrics and is what your healthcare provider tracks. Fetal age (also called embryonic age or conception age) counts from the actual date of fertilisation or conception, making it approximately 2 weeks shorter than gestational age. For example, at 10 weeks gestational age, the embryo is roughly 8 weeks old in terms of fetal age. This calculator uses gestational age by default.