Nutrition of the human fetus during the first trimester--a review.

In all mammalian species nutrition of the conceptus is initially histiotrophic, with the trophectoderm phagocytosing first oviductal and then uterine secretions. Following implantation and establishment of the chorioallantoic placenta there is a transition to haemotrophic nutrition, with exchange between the maternal and fetal circulations. It has long been assumed that the transition occurs soon after implantation in the human, due to the invasive nature of this process. However, the recent realization that the maternal circulation to the placenta is not fully established until the end of the first trimester casts doubt on the validity of this assumption. There is new evidence that the uterine glands discharge secretions into the intervillous space until at least 8 weeks of pregnancy, and that these are taken up by the syncytiotrophoblast. Also, during early pregnancy selected maternal proteins accumulate within the fluid of the coelomic cavity, from which they may be transported to the fetus by the secondary yolk sac. Histiotrophic nutrition may be advantageous to the fetus during the first trimester as it provides nutrients under a low oxygen concentration, so reducing the risk of free radical mediated damage during the sensitive period of organogenesis. Once this is complete, and fetal oxygen requirements rise, there is a transition to haemotrophic nutrition at the start of the second trimester, when the maternal placental circulation is fully established.