trimester

noun

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
Examples are automatically compiled from online sources to show current usage. Read More Opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback.
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 See All Example Sentences for trimester

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

Time Traveler
The first known use of trimester was in 1821

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Cite this Entry

“Trimester.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/trimester. Accessed 17 Oct. 2025.

Medical Definition

trimester

noun
tri·​mes·​ter (ˈ)trī-ˈmes-tər How to pronounce trimester (audio) ˈtrī-ˌ How to pronounce trimester (audio)
: 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
Last Updated: - Updated example sentences
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