Nutrition of the human fetus during the first trimester - A review

In all mammalian species nutrition of the conceptus is initially histiotrop hic, with the trophectoderm phagocytosing first oviductal and then uterine secretions. Following implantation and establishment of, the chorioallantoi c placenta there is a transition to haemotrophic nutrition, with exchange b etween the maternal and fetal circulations. It has long been assumed that t he transition occurs soon after implantation in the human, due to the invas ive nature of this process. However, the recent realization that the matern al circulation to the placenta is not fully established until the end of th e first trimester casts doubt on the validity of this assumption. There is new evidence that the uterine glands discharge secretions into the intervil lous space until at least 8 weeks of pregnancy. and that these are taken up by the synchrotrophoblast. 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 n utrition ma! be advantageous to the fetus during the first trimester as it provides nutrients under a low oxygen concentration, so reducing the risk o f free radical mediated damage during the sensitive period of organogenesis . Once this is complete, and fetal oxygen requirements rise, there is a tra nsition to haemotrophic nutrition at the start of the second trimester, whe n the maternal placental circulation is fully established. (C) 2001 IFPA an d Harcourt Publishers Ltd.