A pregnancy due date (Estimated Due Date or EDD) is calculated from the first day of your last menstrual period (LMP) or from a dating ultrasound. Most pregnancies last about 40 weeks from LMP.
Method 1: Naegele's Rule (Most Common)
EDD = LMP + 9 months + 7 days
Equivalently: add 280 days (40 weeks) to the first day of your LMP.
Example: LMP = 1 January 2025
Add 9 months โ 1 October 2025
Add 7 days โ 8 October 2025
EDD = 8 October 2025
Method 2: 280-Day Count
Simply add 280 days to your LMP date:
| LMP | + 280 days | EDD |
|---|---|---|
| 1 Jan 2025 | + 280 | 8 Oct 2025 |
| 15 Mar 2025 | + 280 | 19 Dec 2025 |
| 1 Jun 2025 | + 280 | 7 Mar 2026 |
Method 3: Ultrasound Dating
An early pregnancy scan (8โ13 weeks) measures the crown-rump length (CRL) of the embryo and provides a gestational age estimate accurate to ยฑ3โ5 days. This is more accurate than Naegele's rule, especially if your cycles are irregular.
Ultrasound EDD is used clinically if it differs from LMP-based EDD by more than:
- 5 days (before 9 weeks)
- 7 days (9โ14 weeks)
- 14 days (after 14 weeks)
Pregnancy Trimesters
| Trimester | Weeks | Key milestones |
|---|---|---|
| First | 1โ12 | Embryo development, first scan |
| Second | 13โ26 | Anatomy scan at 20 weeks |
| Third | 27โ40 | Growth scans, birth preparation |
Expected Date of Delivery (EDD) vs Actual Birth
Only about 4% of babies arrive on their exact due date. Most births occur within 2 weeks either side.
| Week | Percentage of births |
|---|---|
| Before 37 weeks | Preterm (8%) |
| 37โ38 weeks | Early term (19%) |
| 39โ40 weeks | Full term (37%) |
| 41 weeks | Late term (23%) |
| 42+ weeks | Post-term (6%) |
Key Pregnancy Dates to Calculate
From your EDD, you can work backwards to important milestones:
LMP = EDD โ 280 days
End of first trimester = LMP + 84 days (12 weeks)
Anatomy scan (20 weeks) = LMP + 140 days
Full term (37 weeks) = LMP + 259 days
Adjusting for Irregular Cycles
Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is:
- Longer than 28 days: Add the extra days to your EDD
- Shorter than 28 days: Subtract the difference
Example: 35-day cycle (7 days longer than 28):
Adjusted EDD = Naegele's EDD + 7 days