Finding out your estimated due date is one of the most exciting moments in early pregnancy. But how is it actually calculated?
The Standard Method: Naegele's Rule
Named after German obstetrician Franz Naegele (1778–1851), this is the most widely used method:
Due Date = First day of LMP + 280 days (40 weeks)
LMP = Last Menstrual Period
Step-by-Step Example
If your LMP was January 15, 2026:
1. Add 280 days
2. Due date = October 22, 2026
Pregnancy Timeline
| Trimester | Weeks | Key Developments |
|---|---|---|
| First | 1–13 | Conception, embryo formation, heartbeat |
| Second | 14–27 | Fetus grows, movements felt, anatomy scan |
| Third | 28–40 | Rapid growth, lungs mature, position for birth |
How Accurate Is the Due Date?
- Only 4% of babies are born on their exact due date
- 70% are born within 10 days of their due date
- A full-term pregnancy is considered 37–42 weeks
- First babies are more likely to arrive late
- Subsequent babies are more likely to arrive early
Factors That Can Affect Due Date
- Cycle length: Women with longer cycles (35+ days) may ovulate later
- Irregular periods: Makes LMP-based dating less reliable
- Early ultrasound: Dating scan between 8–14 weeks is most accurate
- Multiple pregnancy: Twins/triplets often arrive earlier
Using Our Pregnancy Calculator
Our Pregnancy Calculator provides:
- Estimated due date based on Naegele's rule
- Current week and trimester automatically
- Progress percentage
- Clean, shareable results
💡 Note: Your due date is an estimate. Only 1 in 20 babies are born on their exact due date. Think of it as your "due month" rather than your "due day."
*Calculate your due date with our free Pregnancy Calculator.*
