How accurate is Naegele's rule for due date calculation?

Naegele's rule assumes a 28-day cycle with ovulation at day 14. For cycles longer or shorter than 28 days, the rule may be inaccurate by days to weeks. First-trimester ultrasound (7–13 weeks) with crown-rump length measurement is the most accurate method for gestational age dating.

What is the difference between gestational age and fetal age?

Gestational age (menstrual age) is counted from the first day of the LMP and is 2 weeks longer than true fetal age (fertilization age). All standard obstetric references and growth charts use gestational age.

Estimated Gestational Age (EGA) from LMP

Calculates estimated gestational age and due date using Naegele's Rule: EDD = LMP + 280 days (40 weeks). Alternatively: EDD = LMP + 1 year − 3 months + 7 days. Gestational age in weeks = (today − LMP) ÷ 7. Crown-rump length ultrasound is most accurate before 14 weeks.

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Estimated Gestational Age (EGA) from LMP

Frequently Asked Questions

How accurate is Naegele's rule for due date calculation?

What is the difference between gestational age and fetal age?