Pregnancy trimester dates
Pregnancy is divided into three trimesters, counted in weeks from the first day of the last period. Here is exactly where each one starts and ends.
Sources last checked . How we write and check
First trimester0 to 13 weeks 6 daysAbout months 1 to 3
Second trimester14 weeks 0 days to 27 weeks 6 daysAbout months 4 to 6
Third trimester28 weeks 0 days to birthAbout months 7 to 9
Why sources draw the lines differently
This site uses ACOG’s boundaries: the first trimester through 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days.1 The NHS week-by-week guide groups week 13 with the second trimester.2 Neither changes your due date. What matters clinically is your exact gestational age in weeks and days, so care teams rarely rely on trimester labels.
Your own trimester dates
Enter your due date or last period in the due date calculator. The result lists the calendar date each trimester begins for your pregnancy. For month conversions, see weeks, months and trimesters.
Questions people ask
When does the second trimester start?
At 14 weeks 0 days by ACOG’s definition. Some UK sources, including the NHS week-by-week guide, start it at week 13.1,2
When does the third trimester start?
At 28 weeks 0 days, by ACOG’s definition.1
How long is each trimester?
The first and second trimesters are each 14 weeks long by this definition. The third runs from 28 weeks until birth, which is 12 weeks to the due date but can be longer or shorter.1
Sources
- How Your Fetus Grows During PregnancyAmerican College of Obstetricians and Gynecologists
- Week-by-week guide to pregnancyNHS Best Start in Life
Links checked November 12, 2026. Citing a source does not mean it endorses this site.