trimester

noun

tri·​mes·​ter (ˌ)trī-ˈme-stər  ˈtrī-ˌme- 

1

: a period of three or about three months

especially : any of three periods of approximately three months each into which a human pregnancy is divided

2

: one of three terms into which the academic year is sometimes divided

Did you know?

Semester, which comes from the Latin words for "six" and "month", has come to mean half an academic year when the year is divided into two segments. When an academic year is divided into three segments, each is called a trimester (which is usually a bit more accurate, since each segment often is close to three months in length). Some colleges operate on the "quarter" system, with the summer being the fourth quarter, but this just means that each quarter is basically a trimester. In a human pregnancy, a trimester is three months long, representing one-third of the nine months that a typical pregnancy lasts.

Examples of trimester in a Sentence

development in the first trimester

Recent Examples on the Web

From pull-on pants and drawstring styles to low-rise silhouettes, these are the best maternity pants on the market, from a Vogue writer who’s in the third trimester (and enjoying a lot of macaroni and cheese). —Cortne Bonilla, Vogue, 9 Oct. 2025

Omega-3s accumulate in the brain and retina of the eye during the third trimester of pregnancy. —Brittany Lubeck, Verywell Health, 8 Oct. 2025

The actress, model and entrepreneur had progressed beyond the first trimester. —Julio Cesar Valdera Morales, MSNBC Newsweek, 6 Oct. 2025

An expecting mother’s blood volume rises by 45% and even up to 50-60% percent in the late third trimester to feed the growing fetus. —Elizabeth Yuko, Flow Space, 30 Sep. 2025

Word History

Etymology

French trimestre, from Latin trimestris of three months, from tri- + mensis month — more at moon

First Known Use

1821, in the meaning defined at sense 1

Phrases Containing trimester

• fourth trimester

Rhymes for trimester

• ancestor
• arrester
• digester
• investor
• protester
• requester
• semester
• sequester
• silvester
• ester
• esther
• fester

Medical Definition

trimester

noun

tri·​mes·​ter (ˈ)trī-ˈmes-tər  ˈtrī-ˌ 

: a period of three or about three months

especially : any of three periods of approximately three months each into which a human pregnancy is divided