tri·​mes·​ter (ˌ)trī-ˈme-stər 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
: 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 Additionally, Pfizer's RSV vaccine is also recommended for pregnant people in the third trimester to protect babies. Dr. Jade Cobern, ABC News, 23 Nov. 2023 Babies’ respiratory systems only fully mature during the third trimester of pregnancy, meaning many apnea and breathing disorders often accompany premature deliveries. Andrew Paul, Popular Science, 16 Nov. 2023 Of the ones coming in the first trimester, more than half of them had no plan for prenatal care. Aria Bendix, NBC News, 7 Nov. 2023 An unplanned C-section and ongoing struggles with breastfeeding meant my fourth trimester (the 12 weeks after giving birth, which many experts now believe is as crucial to a mother’s health as their first three trimesters) was filled with unexpected challenges. Liz Hammond, Vogue, 26 Oct. 2023 Advertisement This is the first year when adults age 60 and older and women in their third trimesters of pregnancy have the option of receiving a RSV vaccine, which was approved by the U.S. Food and Drug Administration this summer. Paul Sisson, San Diego Union-Tribune, 6 Nov. 2023 Pregnant people can get immunized during their third trimester to impart protection to their infants. Rachel Murphy, Verywell Health, 5 Nov. 2023 One thing pregnant air travelers should take extra precautions to avoid at any trimester? Melissa Mills, Parents, 14 Sep. 2023 About 1% take place after 21 weeks, and far less than 1% occur in the third trimester and typically involve emergencies such as fatal fetal anomalies or life-threatening medical emergencies affecting the mother. Politifact Staff Writer, Dallas News, 24 Aug. 2023 See More

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'trimester.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History


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

Dictionary Entries Near trimester

Cite this Entry

“Trimester.” Dictionary, Merriam-Webster, Accessed 30 Nov. 2023.

Medical Definition


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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