The presence of a growing fetus in the uterus is an extra-physiological load on the mother and to cope with this extra load the maternal homeostatic regulators of gestation are set at new levels. The pregnant woman experiences adaptive changes in body composition, cardiovascular function and metabolism in order to support the adjustments which enable successful development and delivery of a new individual. This requires that the mother provide an intrauterine environment compatible with satisfying fetal needs. Essential mechanisms relating to the transfer of nutrients and oxygen from the mother to the fetus and the removal of heat, carbon dioxide and nitrogenous wastes from the fetus need to be established. Initially, the maternal organism responds to ovarian hormones during the menstrual cycle in anticipation of implantation and, subsequently, it adjusts to hormonal and neuronal signals originating from the maternal, placental and fetal compartments to synchronize the progress of development. The placenta is an extra source of hormones to supplement the neuroendocrine and immune systems. At the end of gestation, homeostatic controls are critical in mediating maternal–fetal wellbeing because parturition involves the sudden disruption of the maternal–fetal exchange system. At birth the commitment to reproduction is continued and involves a different set of adjustments resulting in lactation and parental behaviour.

THE PLACENTA AS A MATERNAl-FiTAL INTERFACE

The placenta is an organ designed for the transfer of heat and matter between the maternal and fetal blood flows and, as such, is a sophisticated example of biological devices known as exchangers. As described in Chapter 8, the original trophoblastic capsule becomes underlaid with mesoderm and is transformed into the chorion which becomes part of the placenta.