How trimesters are defined
| Trimester | Weeks | Days from LMP |
|---|---|---|
| First | week 1 to 13 weeks 6 days | 0–97 |
| Second | 14 weeks 0 days to 27 weeks 6 days | 98–195 |
| Third | 28 weeks 0 days until birth | 196+ |
The trimester calculator counts from the first day of your last period (or works back from your due date) and gives the calendar dates for each trimester, highlighting the one you are in now.
What each trimester involves
The first trimester is when the major organs form; many people have early symptoms and a dating scan. The second trimester brings rapid growth, the anatomy scan and usually the first movements. The third trimester is about growth and getting ready for birth, with more frequent appointments. See the Pregnancy Timeline for every week.
Key dates in each trimester
- First trimester: a positive home test around 4 weeks, the first prenatal appointment (often 8–10 weeks), and first-trimester screening options around 10–13 weeks.
- Second trimester: second-trimester screening (about 15–22 weeks), the anatomy scan (about 18–22 weeks) and glucose screening (commonly 24–28 weeks).
- Third trimester: more frequent appointments, group B strep screening (about 36–37 weeks), early term at 37 weeks, full term at 39 weeks and the due date at 40 weeks.
Timing varies between providers and countries, so use these as a general guide. Your result shows the exact start and end dates of your own trimesters.