These two terms probably belong in the “False Friends 101” category, but are nevertheless sometimes confused in translation. Both trimestre and “trimester” refer to a period of three months. In English “trimester” is perhaps most often used to refer to stages of human gestation: “the first trimester of pregnancy” (el primer trimestre del embarazo). In addition, many US universities divide the academic year into three trimesters (fall, winter and spring), each having a duration of approximately 12 weeks.
However, in legal, business or accounting contexts a three-month period is called a “quarter” (i.e., la cuarta parte de un año). Thus, in these contexts trimestre must be rendered as “quarter” rather than as “trimester.” To cite a few simple examples, the accounting year (ejercicio contable) is divided into “quarters,” (trimestres); VAT (IVA) is paid “quarterly” (trimestralmente), and business profits are typically announced in a “quarterly earnings report” (informe trimestral de resultados).