The word trimester means "3 months." This can be confusing, because a normal pregnancy is 40 weeks, a bit longer than 9 months. Yet, health care providers and pregnant women often use trimesters to give pregnancy a beginning, middle, and end. In fact, most providers talk about pregnancy in weeks, rather than months or trimesters.

The first trimester takes place from conception through week 14. The second trimester is from week 14 through week 28. And the third trimester is from week 28 through labor and deliver. Of course, delivery varies a lot, but it averages at week 40.