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

Ectopic pregnancies, while rare, have long been recognized as the leading cause of maternal deaths in the first trimester. — Andrea Suozzo, ProPublica, 9 Sep. 2026

The researchers also analyzed data from 1,210 girls in the Copenhagen Mother-Child Cohort who were followed from infancy to adolescence; their mothers reported paracetamol use during the third trimester. — Jen Christensen, CNN Money, 8 Sep. 2026

Additionally, pregnant women who are at a heightened risk of contracting HIV should be tested at the beginning of their pregnancy and again during the third trimester, says the CDC. — Tj Rhodes, Sioux Falls Argus Leader, 26 Aug. 2026

According to data published in February by the Centers for Disease Control and Prevention, about 1 in 4 mothers did not receive prenatal care beginning in the first trimester in 2024, an increase of 13% from 2021. — Madeline Mitchell, USA Today, 26 Aug. 2026

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