Pregnancy Due Date Formula (Naegele's Rule)
Calculate the estimated due date from the last menstrual period.
The standard method used by healthcare providers.
The Formula
EDD = LMP + 280 days (or LMP + 1 year - 3 months + 7 days)
Naegele's rule estimates the due date by adding 280 days (40 weeks) to the first day of the last menstrual period. Most babies arrive within two weeks before or after this date.
Variables
| Symbol | Meaning |
|---|---|
| EDD | Estimated Due Date |
| LMP | First day of the Last Menstrual Period |
| 280 days | Average human gestation (40 weeks) |
Example 1
LMP was January 1, 2026
Method 1: January 1 + 280 days
Method 2: January 1 + 1 year = January 1, 2027 → subtract 3 months = October 1 → add 7 days
EDD = October 8, 2026
Example 2
LMP was March 15, 2026
March 15 + 1 year = March 15, 2027
Subtract 3 months = December 15, 2026
Add 7 days = December 22, 2026
EDD = December 22, 2026
When to Use It
Use Naegele's rule when:
- Estimating a pregnancy due date from the last menstrual period
- Planning prenatal care appointments and milestones
- Preparing for the arrival window (2 weeks before to 2 weeks after EDD)
- The menstrual cycle is regular (approximately 28 days)