A trimester is a term commonly used in education and pregnancy to divide a period into three equal parts. In an academic setting, a school year may be divided into three trimesters, each lasting about 12 to 13 weeks. This structure allows for more frequent assessments and a varied curriculum throughout the year. In the context of pregnancy, a trimester refers to one of the three stages of gestation, each lasting approximately three months. The first trimester covers weeks 1 to 12, the second from weeks 13 to 26, and the third from weeks 27 to 40. Each trimester is associated with specific developmental milestones for the fetus and changes for the mother.