Estimated due date by Naegele's Rule, with gestational milestones, current trimester and pregnancy progress.
Naegele's Rule, used since 1812, adds 280 days (40 weeks) to the last menstrual period date to estimate the due date. It assumes a 28-day cycle and ovulation on day 14, an average, not a universal rule. In practice, only about 5% of births occur exactly on the calculated due date.
The most common formula to estimate the due date is Naegele's Rule: add 280 days (40 weeks) to the last menstrual period date. This assumes ovulation on day 14 of a 28-day cycle.
Longer or shorter cycles shift the actual time of ovulation and conception. That is why a first-trimester ultrasound, ideally between 8 and 12 weeks, is the most precise method to confirm or revise the estimated due date.
ACOG recommends that the EDD be established by ultrasound when there is a discrepancy of more than 5–7 days between the LMP-based calculation and fetal biometry.
The first trimester (week 0 through 13 weeks and 6 days) is marked by organogenesis: all major organs form. The risk of spontaneous miscarriage is highest during this period.
The second trimester (14 weeks through 27 weeks and 6 days) brings rapid growth and first fetal movements. The anatomy scan at week 20 assesses structure, growth and placental position.
The third trimester (week 28 until delivery) involves lung maturation, weight gain and preparation for birth. Fetal monitoring intensifies as the due date approaches.
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Naegele's Rule adds 280 days (40 weeks) to the last menstrual period date to estimate the due date. It assumes a 28-day cycle and ovulation on day 14. Longer or shorter cycles shift the actual time of ovulation, which is why ultrasound is more accurate for confirming or revising the individual EDD.
Medical notice: this calculation is educational, based on population-level averages, and does not replace prenatal care, ultrasound, diagnosis or guidance from a doctor or obstetrician.
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