trimester

noun

tri·​mes·​ter (ˌ)trī-ˈme-stər  ˈtrī-ˌme- 

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

This was the first year that middle school boys from the school’s Saint Florian Campus joined the Blessed Theresa students on their service day, which is hosted once every trimester, said Gallegos. —Alyssa N. Salcedo, jsonline.com, 10 Nov. 2025

Lawrence, who was in the second trimester of her second pregnancy while filming, shot the movie in Calgary, Canada, with co-stars Robert Pattinson (who plays her hapless husband), Sissy Spacek, Nick Nolte, and LaKeith Stanfield. —Ryan Lattanzio, IndieWire, 7 Nov. 2025

Perri and Costabile welcomed Pixie in October 2022, nearly two years after Perri experienced a third-trimester pregnancy loss. —Kimi Robinson, USA Today, 4 Nov. 2025

The makeup artist wore a long beige robe with puffy sleeves for the photos, commemorating her third trimester. —Rachel Raposas, PEOPLE, 3 Nov. 2025

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

Phrases Containing trimester

• fourth trimester

Rhymes for trimester

• ancestor
• arrester
• digester
• investor
• protester
• requester
• semester
• sequester
• silvester
• ester
• esther
• fester

Medical Definition

trimester

noun

tri·​mes·​ter (ˈ)trī-ˈmes-tər  ˈtrī-ˌ 

: 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