•This study introduces a novel procedure - the Inconsolable Doll Task- and is the first to observe prenatal coparenting behaviors during a stressful interaction context.•Co-parenting couples’ dynamic of negative escalation predicted the child’s cognitive development at 18-months.•These predictions remained significant when accounting for prenatal and postnatal assessments of playful coparental behavioral patterns and self-reported coparental perceptions, as well as when controlling for parental education.•The IDT can be useful in prevention and early intervention of expectant parents. Studies have demonstrated that coparenting can be assessed prenatally through playful observational conditions, including simulated baby enactments. Regrettably, there is a lack of empirical research examining how prenatal coparenting under the emotional stress elicited by the distress of a simulated infant predicts children’s cognitive development. The current longitudinal study introduces a novel procedure—the Inconsolable Doll Task—to assess prenatal coparenting behavioral dynamics under the stress of a non-responsive doll simulator, and examines the extent to which prenatal interaction patterns predict the child’s cognitive development at 18 months. The sample consists of 105 community-based, co-living, expectant fathers and mothers. Data were collected prenatally, at three, six, and 18 months in home and lab visits. Results indicate that the prenatal coparenting dynamic of negative escalation explains a unique variance in children’s cognitive development at 18 months. This effect is evident even when accounting for both prenatal and postnatal assessments of low-stress coparenting behavioral patterns or self-reported coparenting perceptions, and when controlling for parental education. These findings are discussed in terms of their methodical, empirical, and clinical implications.