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" )