Trimester: In obstetrics, one of the three divisions of three months each during pregnancy, in which different phases of fetal development take place. The first trimester is a time of basic cell differentiation. The second trimester is a period of rapid growth and maturation of body systems. A second-trimester fetus that is born prematurely may be viable, given the best hospital care possible. The third trimester marks the final stage of fetal growth, in which systems are completed, fat accumulates under the soon-to-be-born baby's skin, and the fetus at last moves into position for birth. This trimester ends with birth.