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Naegele’s Rule: The Formula Behind Your Due Date

Published in 1812, Naegele’s Rule is the simple arithmetic formula that forms the basis of almost every pregnancy due date calculator in use today.

Ashok Kumar Poudel
Written by
Ashok Kumar Poudel
Health & Wellness Writer
Dr. Bina Basnet
Medically reviewed by
MBBS, MD — Gynecologist & Obstetrician
Last reviewed:
Feb 2026

The Formula

Naegele’s Rule:

  1. Start with the first day of your last menstrual period (LMP)
  2. Add 1 year
  3. Subtract 3 months
  4. Add 7 days

Equivalent to: LMP + 280 days (40 weeks)

Example: LMP = January 1, 2024
Step 1: Add 1 year → January 1, 2025
Step 2: Subtract 3 months → October 1, 2024
Step 3: Add 7 days → October 8, 2024 (EDD)

History & Origin

Franz Karl Naegele (1778–1851) was a professor of obstetrics at the University of Heidelberg, Germany. He observed that pregnancies lasted approximately 10 lunar months from the last menstrual period, and formalized this observation into the simple rule that bears his name. Published in his 1812 textbook, the rule quickly spread through European medicine.

Remarkably, this 200-year-old formula remains the global standard. Modern evidence supports a mean gestation of 280 days from LMP (though with significant individual variation), validating Naegele’s original observation.

Cycle Length Adjustment

Naegele’s original rule assumes a 28-day menstrual cycle with ovulation at day 14. For women with different cycle lengths, the modified rule adds an adjustment:

EDD = LMP + 280 + (your cycle length − 28) days

So if you have a 35-day cycle: EDD = LMP + 287 days.
If you have a 21-day cycle: EDD = LMP + 273 days.
Our calculator handles this adjustment automatically.

Limitations of Naegele’s Rule

  • Assumes ovulation at day 14 (varies widely between women)
  • Less accurate for irregular cycles
  • Requires accurate recall of LMP date
  • Does not account for individual biological variation in gestation length
  • Some researchers suggest the rule slightly overestimates gestation in first-time mothers

Despite these limitations, ACOG recommends Naegele’s Rule as the starting point, confirmed and refined by first-trimester ultrasound dating.

Frequently Asked Questions

Common questions answered by our medical team

Naegele's Rule was developed by Franz Karl Naegele (1778–1851), a German obstetrician. He published the rule in 1812, basing it on observations that human gestation averages approximately 10 lunar months (280 days) from the LMP.
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