What Is A Trimester?:

The word trimester means "3 months." This is often a source of confusion, because a normal pregnancy is 40 weeks, a tad longer than 9 months. Nevertheless, health care providers and pregnant women talk about 3 trimesters to help give pregnancy a beginning, middle, and end. Most providers actually talk about your pregnancy in weeks, rather than months or trimesters.

Most commonly, the first trimester is defined as 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, which varies considerably but averages at week 40.