A "trimester" is a term commonly used to describe a period of three months. It is often used in educational settings, where an academic year may be divided into three trimesters, allowing students to take different courses throughout the year. Each trimester typically lasts about 12 to 13 weeks. In the context of pregnancy, a trimester refers to one of the three stages of gestation. The first trimester lasts from conception to about 12 weeks, the second from 13 to 26 weeks, and the third from 27 weeks until birth. Each trimester is associated with specific developmental milestones for the fetus and changes for the mother.