Fetal adaptations to prenatal maternal stress may confer high risk for childhood behavior problems, potentially operating via dynamic fluctuations in infants’ emotions during mother–infant interactions. These fluctuations over time may give rise to behavior problems. Among a sample of 210 low-income mothers of Mexican origin and their 24-week-old infants, dynamic structural equation modeling was used to examine whether within-infant second-by-second emotion processes were predicted by maternal prenatal stress and predicted behavior problems at 36 and 54 months. The mean level around which infant negative affect fluctuated was related to prenatal stress, but not to childhood behavior problems. The volatility in infant negative affect, reflecting greater ebb and flow in infant negative affect during playful interaction, was predicted by prenatal stress and predicted enduring behavior problems in childhood. Results highlight a potential child-driven pathway linking prenatal exposure with childhood behavior problems via infant negative emotional volatility.