Previous studies have found that stressful events during pregnancy can influence the developing fetus, resulting in attentional and neuromotor problems. This prospective study examined whether periods of vulnerability exist for neurobehavioral impairments associated with prenatal stress, using a nonhuman primate model. Twenty-eight rhesus monkey infants were born to mothers in 3 groups: (1) early gestation stress involving mild psychological stress from gestational days 45-90, (2) mid-late gestation stress from days 90-145, and (3) undisturbed controls. Infants were separated from their mothers on days 4, 9, 15, and 22 (+/- 1) postpartum for growth and neurobehavioral assessments. Results indicated that infants from the early gestation stress condition weighed less than infants from mothers stressed during mid-late gestation. Moreover, whereas both groups scored lower than controls on measures of attention and neuromotor maturity, early gestation stress was associated with more pronounced and more pervasive motor impairments than mid-late gestation stress. These results suggest sensitivity to prenatal stress effects peaks during early gestation, tapering off during mid-late gestation. Clarifying the period of greatest vulnerability to prenatal stress moves toward elucidating the underlying mechanism for prenatal stress effects and may lead to more successful intervention and/or prevention.