Pregnancy Due Date
Definition
The estimated due date (EDD) is the predicted date of delivery, calculated as 280 days (40 weeks) from the first day of the last menstrual period (LMP) using Naegele's rule. The formula is: EDD = LMP + 7 days โ 3 months + 1 year. For example, if LMP is March 1, the due date is approximately December 8. This assumes a regular 28-day menstrual cycle with ovulation on day 14. Only about 4โ5% of babies are actually born on their due date โ most arrive within a 2-week window before or after. Ultrasound dating in the first trimester (crown-rump length measurement) provides the most accurate due date estimation, often superseding the LMP calculation.
Why is Pregnancy Due Date Important?
Understanding Pregnancy Due Date empowers you to take control of your personal health and wellness. Whether you are tracking body composition, planning nutrition, or evaluating fitness metrics, this concept provides the foundation for making informed health decisions backed by science.
Our health calculators make these metrics accessible and easy to compute, giving you instant, evidence-based results so you can focus on achieving your wellness goals rather than crunching numbers.
What is an Estimated Due Date?
The Estimated Due Date (EDD) is the predicted date a baby will be born, calculated as 280 days (40 weeks) from the first day of the mother's last menstrual period (LMP). This is based on the assumption of a regular 28-day cycle with ovulation occurring on day 14.
How to Calculate Due Date
Naegele's Rule (Standard Method)
EDD = LMP + 7 days โ 3 months + 1 year
| If LMP Is | Estimated Due Date |
|---|---|
| January 1 | October 8 |
| March 15 | December 22 |
| June 1 | March 8 (next year) |
| September 20 | June 27 (next year) |
Dating Methods Accuracy
| Method | Accuracy | Notes |
|---|---|---|
| First trimester ultrasound | ยฑ5โ7 days | Most accurate โ based on crown-rump length (CRL) |
| LMP calculation | ยฑ14 days | Assumes regular 28-day cycle |
| Second trimester ultrasound | ยฑ10โ14 days | Less accurate as growth varies more |
| Third trimester ultrasound | ยฑ21 days | Used only if earlier dating unavailable |
Pregnancy Timeline
| Trimester | Weeks | Key Developments |
|---|---|---|
| First | Weeks 1โ12 | All major organs form, heartbeat detectable (week 6), highest miscarriage risk |
| Second | Weeks 13โ26 | Gender determination, baby moves ("quickening"), anatomy scan (week 20) |
| Third | Weeks 27โ40 | Rapid growth, lungs mature, baby moves to head-down position |
Important Statistics
- Only 4โ5% of babies are born on their exact due date
- 80% of babies are born within a 2-week window (38โ42 weeks)
- Full term is 39โ40 weeks; early term is 37โ38 weeks; late term is 41 weeks; post-term is 42+ weeks
- First-time mothers tend to deliver 5โ7 days past their due date on average