Human adolescent pregnancy is characterised by poor pregnancy outcome; the risks of spontaneous miscarriage, prematurity and low birth-weight are particularly acute in girls who are still growing at the time of conception. Studies using an adolescent sheep paradigm demonstrate that in growing females who are overnourished throughout pregnancy, growth of the placenta is impaired resulting in a major decrease in lamb birth-weight relative to adolescent dams receiving a moderate intake. In overnourished dams, maternal insulin and IGF-1 concentrations are elevated from early in gestation and these hormones provide a sustained anabolic drive to maternal tissue deposition (primarily of adipose tissue) at the expense of placental growth. The restriction in placental growth is associated with a reduction in placental hormone secretion; maternal concentrations of progesterone, placental lactogen and pregnancy specific protein B are significantly lower in overnourished dams. The role of these placental hormones in nutrient partitioning is equivocal but progesterone supplementation during early pregnancy enhances fetal growth in the overnourished dams. Similarly, maternal GH concentrations are attenuated in overnourished dams and administration of GH to these animals throughout the period of rapid placental proliferation reduces adipose deposition and enhances uteroplacental mass at day 81 of gestation. Nutritional switch- over studies have demonstrated that high nutrient intakes during the second two thirds of pregnancy are most detrimental to pregnancy outcome. In overnourished dams studied during late pregnancy, absolute uterine and umbilical blood flow rates are reduced and the fetuses are relatively hypoxic, hypoglycemic and hypoinsulinaemic. Growth restriction is also associated with absolute reductions in uterine and umbilical glucose and oxygen uptakes. However, it may prove possible to alter the nutrient transport function of the growth restricted placenta in that the imposition of a catabolic phase during the final third of gestation in previously rapidly-growing dams results in a modest increase in lamb birth-weight.
03 - 04 Dec 2001
Society for Endocrinology