Human placentation from nidation to 5 weeks of gestation. Part I: What do we know about formative placental development following implantation?

The implantation of the blastocyst and early development of the placenta are crucial for the success of implantation and pregnancy. However, the formative stages of human placental development are largely unknown because of their existence in a 'black box' where access to samples is extremely limited for ethical reasons. In this review we discuss our current knowledge of early placental formation from the time of implantation at 3 weeks of gestation to approximately 5-6 weeks of gestation, encompassing both the significant anatomical findings derived from the unique specimens obtained in the mid-20th century and the renewed study of this period over the past 10 years as novel models of implantation have been developed.