ABSTRACT
The placenta develops with the principal function of providing nutrients to the foetus. Any impairment in this process is therefore likely to have a profound impact on pregnancy outcome. Placentation in the human is precocious and complex compared with that in the majority of other mammals, starting from the time of implantation during the second week postconception. It is also a highly invasive process, and by day 10-11 postconception the conceptus has become totally embedded in the uterine wall. This form of interstitial implantation is only seen in the human and the great apes. In the majority of mammals, the conceptus remains within the uterine lumen, either with no invasion or with invasion being restricted to localised points around the surface of the chorionic sac.1 Placentation in most of these species involves the interdigitation of foetal and maternal membranes, with exchange occurring between the capillary networks contained within each. By contrast, in the human the invading trophoblast erodes into the endometrial vessels, releasing the maternal blood to bathe the placental villi. This mode of placentation poses unique challenges, in particular haemodynamic issues and those related to oxygen metabolism associated with tapping into the maternal arterial circulation. There is now considerable evidence that many of the major complications of human pregnancy can be traced back to a defect in this process.2 In this chapter we review recent advances in our understanding of placental development during the first and early second trimesters of pregnancy and relate these to pregnancy outcome.
