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

Boys whose mothers had polycystic ovarian syndrome (PCOS) or elevated testosterone levels during the third trimester of pregnancy appear less physically active by age seven. —Daniella Gray, MSNBC Newsweek, 12 May 2025

Smith is now five months pregnant and recently came out of her first trimester, which was difficult for her due to experiencing prenatal depression and having high levels of Human Chorionic Gonadotropin. —Dominique Fluker, Essence, 11 May 2025

The DTaP vaccine, given in childhood, is 90% effective, and the Tdap vaccine administered to pregnant women in the third trimester reduces pertussis cases in newborns by 77%. —Amar Duggirala, Baltimore Sun, 23 Apr. 2025

Pregnant women should get a dose of Tdap during every pregnancy, preferably during the early part of the third trimester, to help protect the newborn. —Mary Walrath-Holdridge, USA Today, 17 Apr. 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