A trimester was a period of three months in one's pregnancy. Pregnancies could have as many as three trimesters. (VOY: "Renaissance Man") Morning sickness usually ended after the first trimester in Humans. (DS9: "What You Leave Behind")
Eleven hours after his 2365 "conception", Deanna Troi's baby was already halfway through the first trimester in terms of development. (TNG: "The Child")
In 2378, The Doctor, having temporarily taken on the appearance of B'Elanna Torres, called Tom Paris "sweetheart", causing Paris to pronounce that Torres was getting affectionate in her third trimester. (VOY: "Renaissance Man")