The mammalian fetus depends absolutely on its mother for its supply of oxygen and nutrients, and for disposal of heat and chemical excreta. It is largely insulated from the external thermal environment by its location in the uterus, the temperature of which is regulated by maternal systems. To an extent that diminishes with advancing gestation, it is also insulated from variations in maternal nutrient supply by the mother's ability to mobilize endogenous nutrient reserves and other maternal adaptations. However, in preparation for postnatal life, the fetus must develop an independent ability to respond appropriately to external stimuli that are delivered via the maternal system. Thus, as term approaches, fetal homeostatic mechanisms mature and the intimacy of the maternal fetal relationship becomes increasingly apparent for some but by no means all maternal and fetal variables, as indicated by the degree of temporal association between maternal and fetal responses. This article will focus on key elements of the maternal fetal relationship in late pregnancy. Most examples will be from experiments on sheep, the first and most commonly used species for studies of fetal physiology and metabolism in vivo in the conscious intact animal.
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