How Due Date is Calculated
Naegele's Rule: Add 280 days (40 weeks) to the first day of your Last Menstrual Period (LMP). This is the standard method used by obstetricians worldwide.
EDD = LMP + 280 days (40 weeks)
Pregnancy Trimester Timeline
| Trimester | Weeks | Key Events |
|---|---|---|
| First | 1–12 | Organ formation, heartbeat at ~6 weeks |
| Second | 13–26 | Movement felt (~18 wks), gender visible |
| Third | 27–40 | Rapid growth, lung maturation |
How accurate is the due date?
Only about 5% of babies are born exactly on their due date. Most are born within 2 weeks before or after. An early ultrasound (before 14 weeks) gives the most accurate estimate.
What if my cycle is not 28 days?
The standard calculation assumes a 28-day cycle. If your cycle is longer, your EDD may be later; if shorter, it may be earlier. Your doctor will adjust based on ultrasound measurements.