Gestational Changes

Abstract

During normal placentation, three types of trophoblast can be differentiated: two types covering the villi, consisting of an outer layer of syncytiotrophoblast and an inner layer of cytotrophoblast, and the third type, the extravillous intermediate trophoblast, which invades the decidua and forms the trophoblastic shell (Figs. 263, 264). If the blastocyst implants and develops normally, the gestational age can be determined fairly accurately from the villous structure; on the 15th day of gestation, the primary villi, which consist of syncytio- and cytotrophoblast only, begin to develop fetal mesoderm, converting them into secondary villi (Figs. 265, 266). From approximately the 20th day on, capillaries form within the mesoderm, and the villi are then referred to as tertiary villi (Figs. 267, 268). The fetal erythrocytes, at first nucleated, loose their nuclei gradually by the tenth to 12th week of pregnancy.