| trimester | A period of 3 months; one-third of the length of a pregnancy. Origin: L. Trimestris, of three-month duration (05 Mar 2000) |
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| pregnancy trimester, first | Period of pregnancy from the first day of the last normal menstrual period through the completion of 14 weeks (98 days) of gestation. (12 Dec 1998) |
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| pregnancy trimester, second | Period of pregnancy from the beginning of the 15th through the 28th completed week (99 to 196 days) of gestation. (12 Dec 1998) |
| pregnancy trimester, third | Period of pregnancy from the beginning of the 29th through the 42nd completed week (197 to 294 days) of gestation. (12 Dec 1998) |