## Naegele's Rule for Dating Pregnancy **Key Point:** Naegele's rule is the standard clinical method for estimating the expected date of delivery (EDD) by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). ### Naegele's Rule Calculation **Mnemonic:** **ADD 7 DAYS, SUBTRACT 3 MONTHS** (simplified version) 1. Start with the first day of LMP: **15th March 2024** 2. Add 7 days: **22nd March 2024** 3. Subtract 3 months: **22nd December 2024** Alternatively: - LMP = 15th March 2024 - Add 280 days (40 weeks) = **22nd December 2024** ### Assumptions Underlying Naegele's Rule - **Regular 28-day menstrual cycle** - **Ovulation on day 14 of the cycle** - **Conception occurs on the day of ovulation** - **Pregnancy duration from ovulation to delivery = 266 days** - **Total from LMP to delivery = 280 days (266 + 14)** **High-Yield:** Naegele's rule assumes a 28-day cycle. For cycles of different lengths, adjust accordingly: - 35-day cycle: Add 287 days - 21-day cycle: Add 273 days **Clinical Pearl:** Naegele's rule is accurate to within ±2 weeks in ~68% of pregnancies. Ultrasound dating in the first trimester (11–14 weeks) is more accurate (±3–5 days) than LMP-based dating. **Warning:** Naegele's rule is unreliable in: - Women with irregular menstrual cycles - Those using hormonal contraception before conception - Polycystic ovary syndrome (PCOS) - Lactational amenorrhea - In such cases, first-trimester ultrasound is the gold standard for dating.
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