Pregnancy Trimesters are Old School

Traditionally, pregnancy has been divided into three segments, known as trimesters, each roughly spanning 13 weeks. The first trimester covers conception up to about 12–13 weeks, often encompassing key developmental milestones like the formation of major organs. The second trimester, from around week 13 to 27, is typically marked by continued fetal growth and the mother’s noticeable physical changes, such as the onset of a visible bump. Finally, the third trimester—week 28 until birth—represents the final stretch, when the fetus rapidly gains weight and prepares for delivery. These three divisions have long been used as a convenient framework to track fetal development and guide prenatal care.