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

In one study, 32 women in the second and third trimester of pregnancy were given chlorella supplements (6 grams per day) for 12–18 weeks. —Jennifer Lefton, Verywell Health, 24 Nov. 2025

Per the Prelude Growth Partners team, Perelel’s prenatals, which feature a different set of ingredients for each trimester, were a significant differentiator. —Emily Burns, Footwear News, 20 Nov. 2025

In the first two trimesters of the pregnancy, cats will often become very friendly and more docile than usual. —Maria Azzurra Volpe, MSNBC Newsweek, 17 Nov. 2025

As a result, women who seek care in the third trimester can be turned away, Sooryakumar said. —Jacqueline Howard, CNN Money, 17 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