Periconceptional undernutrition (UN) in sheep accelerates fetal hypothalamic-pituitary-adrenal (HPA) axis activation, resulting in preterm birth. In contrast, twin conception suppresses fetal HPA function and delays prepartum HPA activation. We hypothesized that these dissimilar effects on fetal HPA activity result from different influences of maternal glucocorticoid (GC) on maturation of the fetal HPA axis, mediated via different activities of placental 11β-hydroxysteroid dehydrogenase (11βHSD) isozymes. We examined the effects of twinning and maternal periconceptional UN from 60 days before until 30 days after mating on the ontogeny of placental 11βHSD-1 and -2 enzyme activities. At day 85 of gestation, placental 11βHSD-2 activity was lower in UN than in normally nourished (N) fetuses (P <.05) and was higher in twins than in singletons (P <.05). Furthermore, placental 11βHSD-1 activity was not different between nutritional groups but was higher in twins than in singletons (P = .01). At day 85, fetal plasma cortisol (P <.001) and cortisone (P <.001) concentrations were lower in UN than in N fetuses, but the cortisol to cortisone ratio was higher in UN than in N fetuses (P = .01). There was no effect of fetus number on plasma cortisol or cortisone concentrations or on the ratio of cortisol to cortisone at day 85. Therefore, periconceptional UN and twinning may result in the alterations of placental 11βHSD isozyme activities at particular times during gestation. Changes in these activities during critical periods of fetal development could affect transplacental transfer or placental generation of GCs that reach the fetus, potentially influencing the timing of activation of the fetal HPA axis, fetal maturation, and hence the development and health later in life.