tri·​mes·​ter | \ (ˌ)trī-ˈme-stər How to pronounce trimester (audio) , ˈtrī-ˌ \

Definition of trimester

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

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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 The procedure is allowed in the third trimester only if life or health is at serious risk, as certified by three doctors. Laura Vozzella, Washington Post, 24 June 2022 Most had mild cases and most infections occurred in the third trimester. Globe Staff,, 19 June 2022 The 9/11 babies were also impacted in the womb, with those in the third trimester having significantly lower cortisol levels. Rachel Yehuda, Scientific American, 18 June 2022 Many of the pregnancies were planned, but doctors did not detect abnormalities until the third trimester. Los Angeles Times, 10 Mar. 2022 Florida law currently bans abortions in the third trimester. Melissa Alonso, CNN, 4 Mar. 2022 The risk of severe COVID-19 is particularly acute in the third trimester and increases the prospect of premature and still birth. Fortune, 23 Dec. 2021 For example, people born in the fall of 1918 were 50 percent more likely than others to develop kidney disease; that may reflect an exposure to the pandemic in the third trimester, while the kidneys were still developing. NBC News, 20 Dec. 2021 Many common cancer treatments run the risk of severely damaging a growing fetus or being harmful to a woman's reproductive system, especially in the first trimester of pregnancy. Maryalice Parks, ABC News, 19 July 2022 See More

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'trimester.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

First Known Use of trimester

1821, in the meaning defined at sense 1

History and Etymology for trimester

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

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Time Traveler for trimester

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The first known use of trimester was in 1821

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Dictionary Entries Near trimester

trimesic acid



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Last Updated

6 Aug 2022

Cite this Entry

“Trimester.” Dictionary, Merriam-Webster, Accessed 13 Aug. 2022.

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More Definitions for trimester


tri·​mes·​ter | \ (ˈ)trī-ˈmes-tər How to pronounce trimester (audio) , ˈtrī-ˌ How to pronounce trimester (audio) \

Medical Definition of trimester

: 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


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