Nutritional backgrounds prior to pregnancy may interact with subsequent gestational intake to influence pregnancy outcome, particularly in young, growing adolescents. To investigate this interaction, singleton pregnancies were established in two groups of adolescent sheep of identical age but different initial weight and adiposity score, classified as good (G) and poor (P) body mass index (BMI). Thereafter, ewes were offered either an optimal control (C) intake to maintain adiposity throughout pregnancy, undernourished (UN) to maintain weight at conception but deplete maternal body reserves, or overnourished (ON) to promote rapid maternal growth and adiposity, resulting in a 2 × 3 factorial design. Gestation length was independent of BMI and reduced in ON dams. Average placental and lamb birth weights were influenced by initial BMI (G > P) and gestational intake (C > UN > ON), with the highest incidence of growth restriction in ON groups. Metabolic challenges at two thirds of gestation revealed enhanced insulin insensitivity in ON dams (higher glucose postinsulin challenge and higher insulin postglucose challenge), but nevertheless fetal growth was constrained. Initial colostrum yield, total IgG, and nutrient supply were reduced in ON groups, but these low-birth-weight lambs exhibited rapid catch-up growth to weaning. Thus, both maternal BMI at conception and gestational intake have a profound influence on pregnancy outcome in young, putatively growing adolescent sheep and may have implications for the nutritional management of pregnant adolescent humans.