The competition for nutrients when pregnancy coincides with continuing growth in biologically immature adolescent girls increases their risk of preterm delivery and low birthweight and is partly replicated in the overnourished adolescent sheep paradigm. Although overfeeding to promote rapid maternal growth robustly leads to a reduction in average birthweight relative to slow-growing control-fed adolescents of equivalent age, the extent of prenatal compromise is variable. This retrospective analysis of a large cohort of identically managed pregnancies determined whether maternal anthropometry predicts the severity of fetal growth-restriction (FGR) in growing adolescents. Singleton pregnancies were established by embryo transfer in adolescents subsequently control-fed (n = 96) or overnourished. The latter pregnancies were classified as non-FGR (n = 116) or FGR (n = 96) if lamb birthweight was above or below the optimally fed control mean minus 2SD. A similar approach categorised placental growth-restriction (PlGR) and preterm delivery. Gestation length, placental mass and lamb birthweight were FGR < non-FGR < control (post hoc P < 0.01). Relative to the non-FGR group, overnourished dams with FGR were marginally leaner and lighter at conception (P = 0.023/P = 0.014) and had greater gestational weight gain (GWG) during the first-third of pregnancy (P < 0.001). GWG during this early period was also higher in PlGR compared with non-PlGR, and in very preterm vs term deliveries (P < 0.01). Likewise maternal leptin concentrations (fat accrual biomarker) were FGR > non-FGR by day 60, and changes in leptin throughout pregnancy predicted attenuated fetal cotyledon mass and birthweight (P = 0.01 to <0.001). The anthropometric antecedents of FGR in still-growing adolescent sheep originate in early pregnancy coincident with early placental development.