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

By contrast, one of the most robust studies on SSRI use in pregnancy found no significant increase in congenital heart defects or most other major malformations when SSRIs were used in the first trimester, adds Dr. Batista. —Sherri Gordon, Parents, 5 Aug. 2025

Others choose to wait until after the first trimester, after the first midwife appointment, or after the first ultrasound, which usually takes place between 10 to 14 weeks into pregnancy, according to Pampers. —Rachael O'Connor, MSNBC Newsweek, 30 July 2025

At that time, Mollie Keen was diagnosed with an incompetent cervix, which is when the lower part of the cervix begins to open (dilate) too early, typically in the second trimester, the release shared. —Melissa Rudy, FOXNews.com, 28 July 2025

That number is lower for patients with higher BMIs, but weight loss during pregnancy, especially in the third trimester, can adversely affect the baby. —Katherine Weaver, Chicago Tribune, 5 July 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