The epithelial bilayer of the fetal placenta consists of villous trophoblasts, which have important functions during pregnancy, e.g. the production of hormones. There are three types of trophoblast cells, cytotrophoblasts, syncytiotrophoblasts and intermediate trophoblasts. Cytotrophoblasts are stem-like cells, because they proliferate and differentiate. Once their proliferation is downregulated, they form syncytiotrophoblasts by fusion. These multinuclear cells surround the villi of the placenta and produce important hormones that are relevant to pregnancy. If there is a disturbance in the cellular growth of cytotrophoblasts, these cells might not be able to fuse. This could lead to pathological pregnancies. It has been shown, that there is a correlation between trisomy 21 and the fusion disorder of cytotrophoblasts. Also, rare malign tumors, like choriocarcinomas can occur by uncontrolled growth of trophoblasts. In this thesis the molecular control of trophoblasts growth are described. In addition, the putative molecular mechanisms underlying the pathologies trisomy 21 and choriocarcinomas are discussed.