Maternal vascularisation of the human placenta: does the embryo develop in a hypoxic environment?

Compared to other mammals, implantation in the human establishes a precocious and intimate apposition between the maternal and fetal tissues. In the past it has been assumed that this relationship permits early onset of haemotrophic exchange, which in turn confers evolutionary advantage by supporting higher development. However, there is now strong evidence from a number of different disciplines to suggest that human pregnancy comprises two contrasting periods. During the first trimester there is little maternal bloodflow to the placenta, the oxygen tension within the feto-placental unit is low, and the uterine glands may provide much of the nutrient supply. At the start of the second trimester the maternal circulation within the intervillous space becomes fully established, the oxygen tension rises and haemotrophic nutrition becomes dominant. During the transition period there is a period of placental oxidative stress, and the response of the tissues to the changing oxygen concentration may play a pivotal role in determining the success or otherwise of the pregnancy.