Hypothalamic-pituitary-adrenal (HPA) activation during pregnancy is linked to dysfunctional behavioral outcomes in the offspring. According to Belsky's differential susceptibility hypothesis, individuals vary regarding their developmental plasticity. Translating the differential susceptibility hypothesis to the field of fetal programming, we hypothesize that infants' temperament, as the constitutionally based reactivity to stimulation, moderates prenatal environmental effects on postnatal emotion regulation.; Maternal HPA axis activity and stress-reactivity during pregnancy was estimated, by measuring cortisol concentrations in saliva, collected at 0, 30, 45 and 60 min after awakening and in blood, collected during a laboratory stress test (Trier Social Stress Test), respectively. Newborns reactivity to stimulation was evaluated between postnatal day 10 and 14 using the Neonatal Intensive Care Unit Network Neurobehavioral Scale. Infant's self-quieting-activities, as an indicator of emotion regulation, were evaluated at the age of six months during the still face paradigm.; Maternal cortisol reactivity to stress during pregnancy was associated with infant's emotion regulation at the age of six months. Whereas cortisol levels after awakening in mid and late pregnancy were not associated with emotion regulation. Furthermore, regression analyses revealed that in interaction with neonatal reactivity, both, prenatal maternal HPA activity as well as prenatal maternal HPA reactivity to stress predicted emotion regulation.; The findings indicate that newborns' reactivity to stimulation is moderating the association between prenatal exposure to maternal glucocorticoids and emotion regulation in infancy. Data suggests that temperamental characteristics of the newborn are a relevant differential susceptibility factor with regard to prenatal effects on emotion regulation.