The Science of Estimating Due Dates: Naegele’s Rule
A standard human pregnancy lasts approximately 40 weeks (280 days) from the first day of the last menstrual period (LMP). Because the exact date of conception is rarely known with certainty, obstetricians globally use the Last Menstrual Period (LMP) as the baseline starting marker.
To calculate the Estimated Date of Delivery (EDD), the standard clinical method is Naegele’s Rule. This mathematical model assumes a regular 28-day menstrual cycle and ovulation occurring on day 14. The formula is:
$$\text{EDD} = \text{First Day of LMP} + 7 \text{ Days} - 3 \text{ Months} + 1 \text{ Year}$$
For example, if the first day of your LMP was October 10, 2025: adding 7 days gives October 17, subtracting 3 months gives July 17, and adding 1 year projects a due date of July 17, 2026.