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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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

During the first trimester of pregnancy, your blood volume increases, so your kidneys have to process extra fluid, the Mayo Clinic explains. Korin Miller, SELF, "7 Ways to Make Sleep More Comfortable When You’re Pregnant," 31 Jan. 2019 The Bump reports that, according to the American Congress of Obstetricians and Gynocologists, women are generally safe to use air travel into their third trimester, but not after 36 weeks. Amy Mackelden, Harper's BAZAAR, "Meghan Markle Arrives in Morocco Wearing the Most Stunning Valentino Cape Dress," 23 Feb. 2019 But after the capital legalized first-trimester abortions on demand in 2007, over half the states passed right-to-life bills declaring that life begins at conception. Fox News, "Mexico City bans use of attractive models during city events," 19 Aug. 2018 Jana Kramer is emerging from the first-trimester fog. Jen Juneau,, "Pregnant Jana Kramer Opens Up About Her Rough First Trimester: 'So Many Blood Vessels Broken'," 26 June 2018 Ross Barr, an acupuncturist known for his many celebrity clients, is also reportedly helping her with a regimen in her third trimester. Chloe Foussianes, Town & Country, "Meghan Markle, Like Her Sister-in-Law Kate Middleton, Is Reportedly Planning for a Natural Birth," 6 Feb. 2019 There are very few women who go through three trimesters of pregnancy not expecting to have a child. Jennifer Wright, Harper's BAZAAR, "Anti-Abortion Activists Are Still Defending Forced Birth with Debunked Science," 26 Jan. 2019 The resulting data showed that the athletes, many of whom had trained into their second trimesters or beyond, had experienced healthy pregnancies and few delivery complications. Gretchen Reynolds, The Seattle Times, "Vigorous exercise, even a trek up Everest, may be safe during pregnancy," 26 Oct. 2018 Think, for example, how the technologies of ultrasound and neo-natal care have shifted the discussion about elective abortions after the first trimester. Chris Stirewalt, Fox News, "Putin or the Democrats? For some today it's not so easy," 19 July 2018

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.

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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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Last Updated

17 Mar 2019

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The first known use of trimester was in 1821

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English Language Learners Definition of trimester

: a period of three months especially : one of three periods into which a woman's pregnancy is often divided
US : one of three periods into which an academic year is sometimes divided


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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Rhyming Dictionary: Words that rhyme with trimester

Spanish Central: Translation of trimester

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