Pregnancy Due Date Formula (Naegele's Rule)
Reference for Naegele's Rule: add 280 days to LMP for EDD.
Covers EDD = LMP + 9 months + 7 days formula, trimester milestones, and ultrasound adjustments.
Need to calculate, not just reference? Use the interactive version.
Open Pregnancy Due Date Calculator →
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)
Key Notes
- Naegele's rule assumes a 28-day cycle with ovulation on day 14 — women with longer cycles may ovulate later, pushing the actual due date further out; the rule is least reliable for irregular cycles
- Only about 4% of babies are born exactly on the EDD; the realistic "due window" is ±2 weeks (gestational weeks 38–42); a birth before week 37 is preterm, and after week 42 is post-term
- First-trimester ultrasound is more accurate than the LMP method (±3–5 days vs ±2 weeks) — when ultrasound dating differs from LMP dating by more than 7 days, clinical practice typically revises the EDD to the ultrasound date
- Gestational age is counted from LMP, not conception — fertilization usually occurs about 2 weeks after LMP, so the formula adds 40 weeks from LMP even though actual fetal age (from conception) is about 38 weeks