trimester /trīmes″tər, trī″-/ [L, trimestris, three months] , one of the three periods of approximately 3 months into which pregnancy is divided. The first trimester includes the time from the first day of the last menstrual period to the end of 12 weeks. The second trimester, closer to 4 months in length than 3, extends from the twelfth to the twenty-eighth week of gestation. The third trimester begins at the twenty-eighth week and extends to the time of delivery.
