trimester

American  

[trahy-mes-ter, trahy-mes-] / traɪˈmɛs tər, ˈtraɪ mɛs- /

noun

• trimesters
  plural

1. a term or period of three months.

2. one of the three approximately equal terms into which the year is divided by some colleges, schools, etc.

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trimester British  

/ traɪˈmɛstə /

noun

1. a period of three months

2. (in some US and Canadian universities or schools) any of the three academic sessions

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Other Word Forms

Derived Forms

• trimestral
  adjective
• trimestrial
  adjective

Inflected Forms

Nouns

• trimesters
  plural

Etymology

Origin of trimester

1815–25; < French trimestre < Latin trimē ( n ) stris of three months, equivalent to tri- tri- + -mē ( n ) stris ( see semester)

Explanation

When something lasts for three months, you can say it lasts for a trimester. Each three-month period of pregnancy, for example, is one trimester. A trimester is typically three months, including a trimester of human gestation (or the time pregnancy lasts), although sometimes it's a length of time divided into three equal parts. An academic calendar is usually split into two semesters, but when it's made up of three sections, each is a trimester. The Latin root of trimester is trimestris, "of three months," from tri-, "three," and mensis, "month."

Example Sentences

See Examples For:

trimester trimesters

It mainly consisted of women inoculated during the third trimester.

From The Wall Street Journal ● Aug. 13, 2026

Researchers use it as an indirect indicator of the balance of estrogen and testosterone a fetus may have been exposed to during the first trimester of pregnancy.

From Science Daily ● Jul. 17, 2026

Her next race was the Houston Marathon in January, when she was in her first trimester and won with a time of 2:24:17.

From BBC ● Apr. 21, 2026

Further, phthalate exposure was measured in the pregnant women from just one urine sample in the first trimester, “which may have introduced exposure misclassification,” the paper notes.

From Slate ● Apr. 16, 2026

I stayed in touch with my parents by phone and saw them on trimester breaks.

From "How Dare the Sun Rise" by Sandra Uwiringiyimana

One study Sufrin conducted found half of states allowed women who were incarcerated to have abortions in the first and second trimesters, but 14% did not allow them at all.

From Salon ● Apr. 7, 2025

But there were increases in white-matter integrity - a measure of the health and quality of connections between brain regions - in the first and second trimesters, which returned to normal levels soon after birth.

From BBC ● Sep. 16, 2024

Fetal brain development occurs throughout pregnancy, but the second and third trimesters are especially critical times, Schantz said.

From Science Daily ● Jan. 3, 2024

Virginia law allows abortion during the first and second trimesters.

From Seattle Times ● Sep. 19, 2023

To qualify for education expense deductions, your expenditures can be made during academic periods that include quarters, trimesters, semesters or when coursework is provided during the summer.

From Encyclopedia.com ● Mar. 31, 2018