Obstetrics

Estimated Due Date (Naegele's Rule)

Calculates estimated due date from last menstrual period.

When to use: Used to establish gestational age and expected delivery date from the first day of the last menstrual period (LMP). Assumes a regular 28-day cycle. Ultrasonography before 14 weeks provides more accurate dating.

Calculator

Days since first day of LMP(days)Enter 0 if LMP started today

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Formula

Naegele's Rule: EDD = LMP + 280 days (40 weeks) Alternatively: LMP date + 9 months + 7 days Assumes regular 28-day cycle with ovulation on day 14

Key Points for NEET PG

Naegele's rule: add 1 year, subtract 3 months, add 7 days to LMP date
Normal gestational age at delivery: 37–42 weeks (term). <37 = preterm, >42 = post-term
First trimester ultrasound (crown-rump length) is more accurate than LMP-based EDD by ±5–7 days
Cycles longer than 28 days: add the difference; shorter: subtract (e.g. 35-day cycle → add 7 days to EDD)
Post-term pregnancy (>42 weeks) associated with increased perinatal morbidity — induce at 41–42 weeks

References

American College of Obstetricians and Gynecologists: Methods for Estimating the Due Date · ACOG Committee Opinion (2017)

For educational purposes only. Not for clinical decision-making without professional oversight.