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 The first trimester of 2020 had already seen a more than 50% increase in deforestation compared to last year, according to INPE data. Jack Guy, Jonny Hallam And Maija Ehlinger, CNN, "Paulinho Paiakan, Amazon chief and indigenous rainforest protector, dies aged 66," 18 June 2020 Data from Brazil's National Institute for Space Research (INPE) shows the first trimester of 2020 saw a more than 50% increase in deforestation compared to last year. Jack Guy And Maija Ehlinger, CNN, "The world lost a football pitch-sized area of tropical forest every six seconds in 2019," 2 June 2020 One patient had a stillbirth in the second trimester and another had a preterm delivery. Jayne O'donnell, USA TODAY, "As states reopen, medical experts recommend pregnant women stay vigilant against COVID-19," 30 May 2020 It's often diagnosed during the second trimester, at which point families are faced with a decision: terminate the pregnancy, or try to carry the fetus to term. Holly V. Hays, The Indianapolis Star, "WISH-TV's Brooke Martin welcomes baby girl 1 year after losing daughter to anencephaly," 27 May 2020 However, the majority of miscarriages happen in the first trimester before the 12th week of pregnancy. Leah Groth,, "Hilaria Baldwin Reveals Miscarriage After 4 Months of Pregnancy—and It's Actually Extremely Rare," 12 Nov. 2019 The study found that prescribing macrolides to pregnant women during the first trimester increased the risk of major malformations to 28 of 1,000 births, compared 18 per 1,000 births with penicillin. Gisela Crespo, CNN, "Common antibiotics linked to increased risk of birth defects, study says," 19 Feb. 2020 Independent clinics are the ones most likely to provide care after the first trimester, the report also found. Stephanie Ebbert,, "The state’s third busiest abortion clinic could close soon without financial support," 6 Jan. 2020 In early March, the journal The Lancet published a report of nine women in Wuhan, the Chinese city where the virus emerged, who tested positive for Covid-19 during their third trimesters. Helen Branswell, STAT, "New reports raise possibility pregnant women can pass coronavirus to fetus, but risk is unclear," 26 Mar. 2020

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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Time Traveler for trimester

Time Traveler

The first known use of trimester was in 1821

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Last Updated

30 Jun 2020

Cite this Entry

“Trimester.” Dictionary, Merriam-Webster, Accessed 7 Jul. 2020.

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How to pronounce trimester (audio)

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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