trimester

American  

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

noun

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

• trimestral adjective
• trimestrial adjective

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

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

Indira has already had the mosquito-borne disease, chikungunya, in the first trimester during a nationwide outbreak in Cuba.

From BBC • Mar. 24, 2026

They note that because the hepatitis B test is typically done in the first trimester, women can still become infected later in pregnancy or some might not get tested at all.

From The Wall Street Journal • Dec. 5, 2025

That last night in 2014, I was also nearing the end of the first trimester of my first pregnancy.

From Salon • Dec. 3, 2025

For starters, Mrs. Richardson had had terrible morning sickness, bouts of dizziness and vomiting that didn’t end with the first trimester but continued on unabated—if anything, more vigorously—as the weeks went on.

From "Little Fires Everywhere" by Celeste Ng