For mother-infant health especially, the pandemic has brought multiple stressors inside a susceptible psychobiological system. We study the longitudinal associations between maternal prenatal and postpartum: (a) COVID-19 stressful events exposure, (b) pandemic psychological stress, and (c) mental health and infants' negative affect. A sample of 643 Italian pregnant women completed a web-based survey from April 8th to May 4th, 2020 and a follow-up at 6 months after delivery. Maternal assessment covered prenatal and postpartum measures for: COVID-19 stressful events exposure, pandemic psychological stress, mental health symptoms (i.e., depression, anxiety, posttraumatic stress disorder) and postpartum, social support and report of infants' negative affect. Maternal mental health symptoms during pregnancy, at the peak of pandemic, is longitudinally associated with infant negative affect, with postpartum mental health mediating this association. Also, maternal COVID-19 stressful events exposure in postpartum is associated with negative affect at 6 months mediated by postpartum mental health symptoms. Maternal pandemic psychological stress during pregnancy predicted mental health symptoms in postpartum. The study supports the association between pandemic-related maternal health across pregnancy and postpartum and offspring's development (i.e., negative affect). It also puts the spotlight on mental health risk in women experiencing lockdown during pregnancy, especially when feeling high psychological stress in pregnancy or when directly exposed to COVID-19 stressful events postpartum.