Human adolescent mothers have an increased risk of delivering low birth weight and premature infants with high mortality rates within the first year of life. Studies using a highly controlled adolescent sheep paradigm demonstrate that, in young growing females, the hierarchy of nutrient partitioning during pregnancy is altered to promote growth of the maternal body at the expense of the gradually evolving nutrient requirements of the gravid uterus and mammary gland. Thus, overnourishing adolescent dams throughout pregnancy results in a major restriction in placental mass, and leads to a significant decrease in birth weight relative to adolescent dams receiving a moderate nutrient intake. High maternal intakes are also associated with increased rates of spontaneous abortion in late gestation and, for ewes delivering live young, with a reduction in the duration of gestation and in the quality and quantity of colostrum accumulated prenatally. As the adolescent dams are of equivalent age at the time of conception, these studies indicate that nutritional status during pregnancy rather than biological immaturity predisposes the rapidly growing adolescents to adverse pregnancy outcome. Nutrient partitioning between the maternal body and gravid uterus is putatively orchestrated by a number of endocrine hormones and, in this review, the roles of both maternal and placental hormones in the regulation of placental and fetal growth in this intriguing adolescent paradigm are discussed. Impaired placental growth, particularly of the fetal component of the placenta, is the primary constraint to fetal growth during late gestation in the overnourished dams and nutritional switch-over studies indicate that high nutrient intakes during the second two-thirds of pregnancy are most detrimental to pregnancy outcome. In addition, it may be 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 pregnancy in previously rapidly growing dams results in a modest increase in lamb birth weight.

Overnourishing adolescent ewes throughout pregnancy promotes maternal tissue synthesis at the expense of placental growth, which in turn leads to a major decrease in lamb birth weight. As maternal dietary intakes are inversely related to peripheral progesterone concentrations in these adolescent dams, it was hypothesized that supoptimal progesterone concentrations in overnourished dams may compromise the growth of the differentiating conceptus resulting in fewer uterine caruncles being occupied and, hence, fewer placentomes formed. This hypothesis was tested by supplementing overnourished adolescent dams with exogenous progesterone during early pregnancy and determining the impact on pregnancy outcome at term. Embryos recovered from superovulated adult ewes inseminated by a single sire were transferred in singleton to the uterus of peripubertal adolescent recipients. After transfer of embryos, ewes were offered a moderate or high amount of a complete diet (n = 11 per group). A further high intake group received a progesterone supplement each day from day 5 to day 55 of gestation (term = 145 days) to restore circulating progesterone concentrations to moderate values throughout the first third of pregnancy (n = 11). For ewes establishing pregnancies (n = 7 per group), live weight gain during the first 100 days of gestation was 66 ± 4, 323 ± 17 and 300 ± 7 g per day, body condition score at term was 2.1 ± 0.05, 3.0 ± 0.08 and 3.1 ± 0.07 units and the duration of gestation after spontaneous delivery was 148 ± 1.7, 144 ± 0.8 and 143 ± 0.8 days for the moderate intake, high intake and high intake plus progesterone groups, respectively. At delivery, fetal cotyledon mass (136 ± 12.1 versus 57 ± 8.2 g, P < 0.001) and lamb birth weight (5164 ± 151 versus 2893 ± 381 g, P < 0.001) were higher in moderate intake than in high intake dams. Progesterone supplementation restored circulating concentrations to moderate values during the first third of gestation. Lamb birth weight in the high intake plus progesterone group (4150 ± 389 g) was intermediate between the high intake (P < 0.02) and moderate intake (P < 0.05) groups, but this change in birth weight was not associated with corresponding changes in fetal cotyledon mass (76 ± 10.3 g). Moreover, the number of fetal cotyledons was similar in all three groups. Thus, progesterone did not directly affect the growth of the fetal cotyledon but may have influenced placental vascularity, blood flow or nutrient transfer capacity or alternatively the development of the embryonic inner cell mass.