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1821, "period of three months," from French trimestre (early 17c.), from Latin trimestris "of three months," from tri- "three" (see tri-) + mensis "month" (see moon (n.)). Specific obstetrics sense is attested from 1916.
trimester tri·mes·ter (trī-měs'tər, trī'měs'-)n. A period of three months.