Definition of trimester

The word "trimester" refers to a period of three months, specifically in relation to pregnancy. The term "trimester" comes from the Latin word "tri-" meaning "three" and the Greek word "-mestros" meaning "to sit," which was a reference to the observation that a fetus typically stops moving around in the uterus during the third trimester. The concept of dividing pregnancy into three equal parts, each lasting around three months, was proposed by the Dutch obstetrician and anatomist Nicolaas Tulp in the 17th century, although the term "trimester" was not commonly used until the 19th century. Today, the first trimester is generally considered to be the first 12 weeks of pregnancy, the second trimester is the period from week 13 to week 27, and the third trimester is from week 28 to week 40.