Trophoblast giant cells (TGCs) have a unique ability to form pseudo-vascular spaces in the placenta. Proper development of these maternal blood spaces is crucial for successful pregnancy outcomes. Unlike blood vessels in other organs, the vascular space in placenta is line by non-endothelial TGCs. Previous work in Cross lab has established that there are multiple subtypes of TGCs that line various segments of maternal vasculature. These subtypes of TGCs differ in location, function, morphology and gene expression pattern. Mouse genetic studies have given us insights into development of the mouse placenta but our understanding of how TGCs behave like endothelial cells to form blood spaces is limited. By developing a three dimensional in vitro system we have demonstrated that of all the TGC subtypes, P-TGCs have an intrinsic ability to form vascular-like cavities. Whereas other subtypes of TGCs such as S-TGCs require low oxygen conditions to differentiate. We also used both in vivo and in vitro models of VEGF over-expression to address role of VEGF signaling on TGC development. A small population of trophoblast progenitors expresses VEGF receptor 2 transiently that responds to the VEGF-A. Misbalance in VEGF signaling leads to altered venous side TGC differentiation and vascular development. Our results revealed that VEGF signaling plays an important role in differentiation of trophoblast cells and patterning of maternal blood spaces. In order to determine role of Prl-related genes in placental development we used a large-scale knockout of 22 Prl-related genes. The analysis of crosses between different parental genotypes revealed that the absence of Prl-related genes in the maternal tissue causes abnormal development of the maternal vasculature in the placenta. The arterial spaces are dramatically larger and invasion of SpA-TGCs is reduced. This suggests that certain decidua-expressed Prl-related genes are crucial for proper development and functioning of precisely TGCs. Overall, the work done in this thesis contributes to our understanding of the vascular development of by TGCs. Various pathologies during human pregnancy are associated with failure of trophoblast cells in establishing proper vasculature. Therefore a better understanding of development of these spaces in the mouse placenta will help us gain insights into human placentation and treatment of pregnancy related diseases.