How Due Dates Are Estimated: Naegele's Rule Explained
How an estimated due date is worked out from the first day of the last period, how pregnancy weeks are counted, and why few babies arrive on the exact day.
By the CalClu team2 min read
An estimated due date (EDD) is a starting point for planning, not a deadline. Here's how it's usually calculated.
Naegele's rule
The classic method counts from the first day of your last menstrual period (LMP):
Example: if your last period started on March 10, the estimated due date is December 15.
A traditional shortcut, adding seven days, subtracting three months and adding a year, gives nearly the same date. It can differ by a day or two depending on the lengths of the months in between.
Counting weeks
Pregnancy is counted from the LMP, which is typically about two weeks before conception. So at conception you're already considered roughly "two weeks pregnant". Weeks are often written like 12+3, meaning 12 weeks and 3 days.
- First trimester: up to the end of week 13
- Second trimester: weeks 14 to 27
- Third trimester: week 28 until birth
Trimester boundaries vary slightly between sources.
When the rule is less accurate
Naegele's rule assumes a 28-day cycle with ovulation around day 14. Longer or irregular cycles shift the real date. Care providers often adjust the estimate using an early ultrasound, which measures the baby's size and is generally considered more reliable for dating early in pregnancy.
Few babies arrive on the day
Only a small share of babies are born on their estimated due date. Most arrive within a couple of weeks either side of it, which is why many people think of a "due month" rather than a single day.
After IVF
Due dates after IVF are worked out from the embryo transfer date and the embryo's age, so the clinic will usually give you a precise estimate.
This guide is general information only. Your midwife or doctor will confirm your due date.
More tools in this area: Pregnancy & Parenting calculators