This document summarizes fetal development from weeks 1 through 40 of pregnancy in 3 month increments. In the first trimester (weeks 1-13), the embryo forms and organs begin to develop. In the second trimester (weeks 14-27), the fetus grows rapidly and movements can be felt. In the third trimester (weeks 28-40), the fetus continues to develop and grow, maturing organs in preparation for birth. Each month provides details on the physical changes and average measurements of the developing fetus as well as common symptoms experienced by the mother.