What Trimester Actually Means
Trimester just means three months, which sounds simple until you do the math. A full pregnancy runs about 40 weeks, closer to nine and a quarter months than a clean nine, so splitting it into three even chunks was never going to work out perfectly. Doctors and midwives use the word anyway because it’s a useful shorthand, a way to talk about pregnancy as having a beginning, a middle, and an end instead of just one long stretch of weeks. In practice, most of your actual appointments will be discussed in weeks, not trimesters or months, since a week is precise in a way “second trimester” just isn’t.
Explore Each Trimester
How the Three Trimesters Break Down
The first trimester runs from conception through about week 13. The second trimester covers roughly week 14 through week 27. The third trimester starts around week 28 and runs until delivery, which averages around week 40 but varies a fair amount from one pregnancy to the next. You’ll sometimes see these boundaries shifted by a week or so depending on the source, that’s normal, there’s no single universally agreed cutoff down to the day.
Each trimester tends to bring its own general shape, even though every pregnancy is different. The first is often the roughest on your body as it adjusts fast, nausea and exhaustion included, while the baby’s basic organs and structures are forming. The second is when many people start feeling more like themselves again, energy comes back for a lot of women, and this is usually when the anatomy scan happens. The third is the home stretch, bigger, heavier, and more tiring again as your body gets ready for labor.
If you’d rather see things broken down month by month instead of by trimester, our Your Pregnancy, Month by Month guide covers the same nine months in smaller pieces, with a due date calculator built right in if you haven’t figured out yours yet.