Separate fields of inquiry indicate (a) that prenatal stress is associated with heightened behavioral and physiological reactivity and (b) that these postnatal phenotypes are themselves associated with increased susceptibility to both positive and negative environmental influences. Collectively, this work supports Pluess and Belsky's (Psychopathology, 2011, 23, 29) claim that prenatal stress fosters, promotes or "programs" postnatal developmental plasticity. Herein, we review animal and human evidence consistent with this hypothesis before advancing the novel idea that infant intestinal microbiota may be one candidate mechanism for instantiating developmental plasticity as a result of prenatal stress. We then review research indicating that prenatal stress predicts differences in infant intestinal microbiota; that infant intestinal microbiota is associated with behavioral and physiological reactivity phenotypes; and, thus, that prenatal stress may influence infant intestinal microbiota in a way that results in heightened physiological and behavioral reactivity and, thereby, postnatal developmental plasticity. Finally, we offer ideas for testing this claim and consider implications for intervention and use of probiotics during early infancy.