How to use this tool
- Enter the first day of the last menstrual period (LMP).
- Change the cycle length if yours is usually longer or shorter than 28 days.
- Read the EDD, gestational age in weeks and days, and trimester.
How the EDD is worked out
Pregnancy is counted from the first day of the last menstrual period (LMP), about two weeks before conception. Naegele's rule adds 280 days (40 weeks) to that date. If your cycles are usually 32 days, ovulation probably happened four days later than in a 28-day cycle, so the due date moves four days later too.
Ultrasound dating
The American College of Obstetricians and Gynecologists (ACOG) and the NHS both use an early ultrasound to confirm or correct the date. A first-trimester scan measuring the baby's crown-rump length is the most accurate method. If it differs from the LMP date by more than about a week, the scan date is usually used.
Trimesters
The first trimester runs to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth. A pregnancy counts as full term from 39 weeks 0 days to 40 weeks 6 days.
Frequently asked questions
What does EDD mean in pregnancy?
EDD stands for expected date of delivery, the estimated due date. It is usually calculated as LMP + 280 days and confirmed by an early ultrasound.
How accurate is a due date calculator?
It gives an estimate. Most babies are born within two weeks of the due date, and an early scan is more precise.
How many weeks pregnant am I?
Count from the first day of your last period. The calculator shows weeks and days as of today.
Why is pregnancy 40 weeks if conception is later?
Dating starts from the last period, about two weeks before conception, so 40 weeks of gestation is about 38 weeks after conception.
What if my cycles are irregular?
LMP dating is less reliable. Your doctor will date the pregnancy by ultrasound instead.