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

"Get it on time. But if you can't, do get vaccinated all the way through the third trimester."

From BBC ● Apr. 17, 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

“As such deserts have expanded, patients in the first trimester have had to drive further, and thus it is unsurprising that early initiation is decreasing.”

From Salon ● Mar. 1, 2026

Scientists have shown that this ratio reflects the balance of estrogen and testosterone a fetus is exposed to during the first trimester of pregnancy.

From Science Daily ● Feb. 10, 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

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

The analysis linked acetaminophen use in the second and third trimesters of pregnancy to modest but significant delays in early language development.

From Science Daily ● Jan. 3, 2024

Virginia law allows abortion during the first and second trimesters.

From Seattle Times ● Sep. 19, 2023

Testing a newborn's bowel movement may be a better measure as it is thought to reflect the mother's use of acetaminophen during the last two trimesters of pregnancy.

From Salon ● Jul. 13, 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