Pregnancy Due Date
EDD from last period
About This Calculator
The estimated due date (EDD) is calculated from the first day of the last menstrual period (LMP) using Naegele's rule, which adds 280 days (40 weeks). Only about 5% of babies are born on their exact due date, but 90% are born within two weeks of it. The EDD guides prenatal care scheduling and medical planning.
Formula
Naegele's Rule: EDD = LMP + 280 days
= LMP + 9 months + 7 days
Alternatively: EDD = LMP − 3 months + 7 days + 1 year
From conception date: EDD = Conception + 266 days
Example Calculation
LMP (Last Menstrual Period) was January 14, 2025
- Add 7 days: January 21
- Add 9 months: October 21, 2025
- EDD = October 21, 2025
Estimated due date: October 21, 2025 (±2 weeks)
Pregnancy Milestones
| Week | Trimester | Milestone |
|---|---|---|
| 4 | 1st | Positive pregnancy test typical |
| 8 | 1st | First prenatal appointment |
| 12 | 1st | End of first trimester; miscarriage risk drops significantly |
| 20 | 2nd | Anatomy scan ultrasound; often learn sex |
| 24 | 2nd | Viability threshold (extreme prematurity) |
| 37 | 3rd | Early term; lungs usually mature |
| 40 | 3rd | Full term / due date |
| 42 | 3rd | Post-term; induction typically considered |
Frequently Asked Questions
How accurate is the due date calculation?
Only 4-5% of babies are born on their exact due date. A normal pregnancy lasts 38-42 weeks. The EDD is a midpoint estimate, not a guaranteed date. Early ultrasound (before 12 weeks) is the most accurate dating method and may shift the due date.
What does 'weeks pregnant' mean?
Pregnancy is counted from the first day of the last menstrual period, not from conception. When a woman is '4 weeks pregnant,' conception likely occurred about 2 weeks earlier. The 40-week total includes 2 weeks before conception actually occurs.
What is the difference between gestational age and fetal age?
Gestational age (what doctors use) counts from the LMP. Fetal age (actual age of the baby) counts from conception, which is typically 2 weeks after LMP. A baby described as '20 weeks gestational age' is actually about 18 weeks old from conception.
What if I don't know my LMP?
An ultrasound (especially before 12 weeks) can estimate gestational age very accurately from the size of the embryo/fetus. After 20 weeks, ultrasound dating becomes less precise. IVF pregnancies have exact conception dates, making dating straightforward.