How Doctors Calculate Estimated Date of Delivery (Naegele's Rule)
In obstetrics, determining the precise expected delivery date is vital for scheduling prenatal screenings (NT scan, anomaly scan) and monitoring fetal growth curves:
- Naegele's Formula: EDD = First day of LMP + 1 year − 3 months + 7 days (or LMP + 280 days).
- Cycle Length Adjustment: If a patient's cycle differs from standard 28 days, add or subtract the difference (e.g. for a 32-day cycle, add 4 additional days).
- Ultrasound Corroboration: In cases of irregular periods or oral contraceptive use prior to conception, the first-trimester dating scan CRL (between 11–14 weeks) is considered the gold standard.