What Is A Trimester?

The word trimester means "three months." This is often a source of confusion, because the length of a normal pregnancy is not exactly nine months long (which comes out to 36 weeks). Rather, a pregnancy due date is calculated from the last menstrual period (LMP) and therefore includes a couple of extra weeks before conception. An average pregnancy is 40 weeks.

The term trimester was most likely created by pregnant women and providers who wanted to give balance to the pregnancy process: beginning, middle and end. It places the pregnancy cycle into three neat buckets, which makes it a little easier to discuss body changes, tests, and risks. Otherwise, there is no real significance to the term trimester and many health care professionals use "weeks" when speaking to their patients.

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/deliver, which varies considerably but averages at week 40.

Review Date: July 11, 2001
Reviewed By: A.D.A.M. editorial.


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