tri·​mes·​ter | \(ˌ)trī-ˈme-stər, ˈtrī-ˌ\

Definition of trimester 

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

Keep scrolling for more

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

Jen had a mastectomy and chemo in her second trimester, then radiation after her healthy daughter was born. Kate Rockwood, Good Housekeeping, "After a Metastatic Breast Cancer Diagnosis, One Mom Is Encouraging Others to Ask the Right Questions," 15 Aug. 2018 Deputy Digital Director Laura Redman found herself in Ireland on a work trip during her first trimester, and still had a blast without joining in at the hotel whiskey tasting. Condé Nast Traveler, "Two Super-Travelers on Everywhere They Went While Pregnant," 5 Nov. 2018 Generally, absent extraordinary circumstances, ICE will not detain a pregnant alien during the third trimester of pregnancy. Jewel Wicker, Teen Vogue, "Pregnant Women Are Allegedly Being Denied Medical Care in Immigration Detention Centers," 10 July 2018 The 45% accuracy for the blood test was based on an average of samples taken during the second and third trimester. Jacqueline Howard, CNN, "Blood tests to predict preterm birth risk raise excitement -- and questions," 7 June 2018 Not much is absorbed into the body, and the link is still controversial, but doctors recommend nixing retinoids to be safe, especially during the first trimester. Jenn Sinrich, SELF, "17 Skin-Care and Beauty Products Women Loved When They Were Pregnant," 14 May 2018 During the first trimester, mothers who had children with autism had slightly deeper ultrasounds than women who had typically developing children and children with developmental delays. Laura Sanders, Washington Post, "A new study eases fears of a link between autism and prenatal ultrasounds," 26 Feb. 2018 Nothing is guaranteed, but Kavanaugh will be much likelier to join a decision overturning Roe v. Wade and giving states the ability to ban abortions as early as the first trimester. Dylan Matthews, Vox, "America under Brett Kavanaugh," 11 July 2018 The debate over decriminalizing abortions during the first trimester has polarized Argentina, a country of 44 million. Ryan Dube, WSJ, "Argentina’s Lower House Votes to Ease Abortion Laws," 14 June 2018

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'trimester.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

See More

First Known Use of trimester

1821, in the meaning defined at sense 1

History and Etymology for trimester

French trimestre, from Latin trimestris of three months, from tri- + mensis month — more at moon

Keep scrolling for more

Learn More about trimester

Statistics for trimester

Last Updated

8 Dec 2018

Look-up Popularity

Time Traveler for trimester

The first known use of trimester was in 1821

See more words from the same year

Keep scrolling for more

More Definitions for trimester



English Language Learners Definition of trimester

: a period of three months; especially : one of three periods into which a woman's pregnancy is often divided

: one of three periods into which an academic year is sometimes divided


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

Medical Definition of trimester 

: 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

More from Merriam-Webster on trimester

Rhyming Dictionary: Words that rhyme with trimester

Spanish Central: Translation of trimester

Comments on trimester

What made you want to look up trimester? Please tell us where you read or heard it (including the quote, if possible).


a typical or ideal example

Get Word of the Day daily email!

Test Your Vocabulary

Find the Cousins

  • a-large-tree-with-many-branches
  • Which pair shares a common word ancestor?
True or False

Test your knowledge - and maybe learn something along the way.


Test Your Knowledge - and learn some interesting things along the way.


Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!