Background. Although the negative impact of maternal stress during pregnancy on stress reactivity in offspring is well documented in nonhuman animals, we know little about these relations in humans and their lasting effects. Aim. The authors examined the relation between perceived maternal stress during pregnancy and stability of infant stress reactivity across the first year of postnatal life in humans. Study design. Perceived maternal stress during pregnancy was measured in the immediate postpartum period and again at 10 months after delivery in 84 mothers and examined in relation to behavioral and neuroendocrine stress reactivity measures in their healthy, full-term infants. Outcome measures. Salivary cortisol was collected between 24 to 48 hours of postnatal life in response to a heel stick and again at 10 months of age in response to a toy removal task in the same sample of infants. Behavioral reactivity was coded from direct observation during the toy removal task. Results . Perceived maternal stress during pregnancy and neonatal cortisol reactivity each remained stable across the first 10 months of postnatal life. Maternal stress during pregnancy predicted infant cortisol reactivity at 2 days and 10 months after birth as well as behavioral reactivity at 10 months. Neonatal cortisol reactivity predicted 10-month behavioral reactivity. Conclusion . These preliminary findings suggest that maternal stress during pregnancy may negatively affect neonatal stress reactivity within 24 to 48 hours after birth, and these influences may persist through the first year of postnatal life.