Trimester calculator: when each trimester starts
See the exact calendar dates of your first, second and third trimesters — and which one you’re in today — from your last period or due date. Runs in your browser; nothing is saved.
The first day of bleeding.
Default 28.
How to use it
What weeks are in each trimester?
Pregnancy is split into three trimesters. The most common convention counts by gestational age: the first trimester is 0–13 weeks, the second trimester is 14–27 weeks, and the third trimester is 28 weeks to birth. The calculator turns those week boundaries into real dates for your pregnancy, and marks the one you’re in now. For the milestones inside each, see pregnancy trimesters and milestones.
When does the second trimester start?
The second trimester starts at 14 weeks — often a welcome milestone, as early symptoms tend to ease. Because it depends on your dating, the exact date differs for everyone; enter your details above to see yours.
When does the third trimester start?
The third trimester begins at 28 weeks and runs until your baby arrives, usually around 40 weeks. It’s when monitoring typically increases as the due date approaches — the dates for the key term windows appear in the full OB wheel.
Accuracy
Trimester dates are only as accurate as the dating they’re based on. A first-trimester ultrasound gives the most reliable start point; see how accurate is your due date. Boundaries also vary slightly between sources, so treat the dates as a close guide rather than a hard line.
Frequently asked questions
When does the second trimester start?
The second trimester begins at 14 weeks of gestational age and runs through 27 weeks. Enter your due date or last period above to see the exact calendar date it starts for you.
When does the third trimester start?
The third trimester begins at 28 weeks and continues until birth. The calculator shows the precise date based on your dating.
What weeks are in each trimester?
By the common convention: first trimester 0–13 weeks, second trimester 14–27 weeks, third trimester 28 weeks to birth. Some sources shift the boundaries by a few days.
How long is each trimester?
Each trimester is roughly 13 to 14 weeks. The first is 0–13 weeks, the second 14–27 weeks, and the third 28 weeks until delivery around 40 weeks.
Which trimester am I in?
Enter your last period or due date and the calculator highlights the trimester you’re in today, along with the date ranges for all three.
Last meaningfully updated: 2026-08-25.