Naegele's Rule is the foundational calculation used in obstetrics to determine a patient's Estimated Date of Delivery (EDD) or "due date."
The 280-Day Assumption
Pregnancy is universally tracked in weeks, with a full-term pregnancy lasting 40 weeks (280 days). Crucially, this 280-day clock does not start on the day of conception. Because the exact day of ovulation and conception is rarely known, the clock universally starts on the first day of the Last Menstrual Period (LMP)—approximately two weeks before the woman actually became pregnant.
The Mathematics
In the 1800s, German obstetrician Franz Karl Naegele popularized a simple mental math shortcut to calculate this 280-day interval:
- Identify the first day of the LMP.
- Add 7 days.
- Subtract 3 months.
- Add 1 year (if necessary).
For example, an LMP of January 1st means adding 7 days (Jan 8), subtracting 3 months (Oct 8), resulting in an EDD of October 8th.
EDD &= First day of LMP \ &\quad + 7 days \ &\quad - 3 months \ &\quad + 1 year. (Adjusted for cycle length by adding [Cycle Length - 28] days).
Modern Adjustments
The classic rule assumes a textbook 28-day cycle. However, a woman with a 32-day cycle will ovulate later in the month. The calculator adjusts for this by adding the difference. While early first-trimester ultrasounds are the gold standard for dating a pregnancy, Naegele's Rule remains the starting point for every obstetrical timeline.