trimesters

Trimester is a term used to divide a period into three equal parts. The concept appears in medicine, education, and business, and the exact duration of a trimester can vary by context. Etymologically, trimester derives from Latin tri- "three" and mensis "month," reflecting its original association with three-month intervals.

In obstetrics, pregnancy is divided into three trimesters. The first trimester roughly encompasses weeks 1–12, during which organ formation begins and many pregnancies are established. The second trimester covers weeks 13–26, often marked by fetal movement and an anatomy ultrasound. The third trimester spans week 27 to delivery, with rapid fetal growth and birth planning. Because gestational age, not calendar months, is used, boundaries vary slightly across guidelines. Prenatal care schedules are typically arranged to these divisions, with tests and visits emphasized in each period.

In education, a trimester system divides the academic year into three terms of roughly equal length; instructional focus and assessments occur within each term. Some institutions use three-term calendars instead of semesters; others rely on quarters or year-round schedules. In business and government, trimesters may be used for budgeting or reporting, but quarters are more common.

The term “trimester” implies three-month intervals and is widely used for practical purposes, though exact boundaries are approximate and depend on context.