Pregnancy Weeks Calculator
Calculate how many weeks pregnant you are and your due date from your last menstrual period (LMP). Free pregnancy calculator with trimester breakdown.
Our free pregnancy weeks calculator works out how many weeks and days pregnant you are, your estimated due date (EDD), which trimester you're in, and your approximate conception date — all from the first day of your last menstrual period.
How to calculate pregnancy weeks
Pregnancy is measured from the first day of your last menstrual period (LMP), not from conception. Count the number of days from your LMP to today and divide by 7 to get weeks.
Weeks pregnant = days since LMP ÷ 7
For example, if your LMP was 70 days ago, you are 10 weeks pregnant.
How to calculate your due date
The estimated due date (EDD) is calculated using Naegele's Rule: add 280 days (40 weeks) to the first day of your LMP.
Due date = LMP + 280 days
Only about 5% of babies are born on their exact due date — the EDD is an estimate. Your healthcare provider may adjust it based on an early ultrasound.
The three trimesters
| Trimester | Weeks |
|---|---|
| First trimester | Weeks 1–12 |
| Second trimester | Weeks 13–26 |
| Third trimester | Weeks 27–40 |
When does conception occur?
Ovulation and conception typically occur around day 14 of the menstrual cycle — about two weeks after the LMP. This is why the pregnancy calendar starts two weeks before actual conception.