Emerging evidence suggests that prenatal stress does not solely undermine child functioning but increases developmental plasticity to both negative and positive postnatal experiences. Here we test this proposition using the Norwegian Mother and Child Cohort study while implementing an extreme-group (i.e., high vs. low prenatal stress) design (n = 27,889 children for internalizing and n = 27,892 for externalizing problems). To measure prenatal stress, mothers reported on depressive and anxiety symptoms at gestational weeks 17 and 30 and of stressful life events at gestational week 30. We then evaluated whether, collectively, such prenatal stress amplified the effect of mothers' postnatal depressive and anxiety symptoms on children's internalizing and externalizing behavior problems at age 5 years. Results showed prenatal stress amplified effects of postnatal maternal depression/anxiety on child internalizing but not externalizing behavior, with some indication that this Prenatal-Stress-X-Postnatal-Maternal-Depression interaction proved more consistent with differential susceptibility than diathesis stress thinking: Children exposed to prenatal stress evinced greater internalizing problems if exposed to more postnatal maternal depressive/anxiety symptoms and, somewhat less strongly, displayed less internalizing problems if they experienced lower postnatal maternal depressive/anxiety symptoms. However, analyses using the whole sample instead of extreme groups yielded opposing results with children exposed to the least prenatal stress evincing greater sensitivity to postnatal maternal depressive/anxiety symptoms with regards to externalizing and internalizing behavior. Taken together, it appears that prenatal stress may have differing effects on plasticity depending on prenatal stress severity.