Early life stress (ELS) measured as the exposure to pre- and postnatal maternal self-reported stress and glucocorticoids reportedly predicts higher child negative emotional reactivity and problems in self-regulation and cognition, including executive functioning (EF). However, to date, there is little research available about the independent effects of maternal pre- and postnatal stress and milk cortisol on infant fear reactivity and EF and their moderation by infant sex. 

The aim of this study was to examine the independent effects of pre- and postnatal stress exposure on infant fear reactivity and EF. The study sample comprised 282 (Study I), 391 (Study II), 65 (Study III) and 214 (Study IV) mother-infant dyads from the FinnBrain Birth Cohort Study. The influence of maternal self-reported anxiety and depressive symptoms and the trajectory of symptoms and milk cortisol on infant fear reactivity at 6 and 8 months was examined in Studies I, II and III. The effect of maternal self-reported symptoms on infant EF at 8 months was investigated in Study IV. 

The association between maternal self-reported prenatal stress and higher mother-reported infant fear reactivity in both sexes approached significance. Both maternal prenatal stress and milk cortisol concentrations were associated with higher observed fear reactivity in girls. In turn, the trajectory of continuously increasing maternal stress was associated with lower observed fear reactivity in girls. Furthermore, maternal postnatal anxiety predicted poorer EF in girls, while prenatal anxiety predicted a performance difference between girls and boys. 
The findings of the present study suggest that different forms of ELS might affectt the aspects of infant reactivity and self-regulation, and that these effects are moderated by infant sex.