To investigate the associations between maternal mental health disorders before and during pregnancy and perinatal outcomes and child healthcare utilization between 6 and 18months of age. Among the 6814 mother-child pairs from the Italian Internet-based NINFEA birth cohort, maternal depression, anxiety, and sleep disorders diagnosed by a physician before and during pregnancy were assessed through self-reported questionnaires completed during pregnancy and 6months after delivery. Perinatal outcomes (preterm birth, birth weight, small for gestational age, congenital anomalies, and neonatal intensive care unit (NICU)) and children's healthcare utilization (emergency department (ED) visits, hospitalizations, and outpatient visits) were reported by mothers at 6 and 18months postpartum. We used regression models adjusted for maternal age, education, parity, country of birth, region of delivery, and household income. Maternal mental health disorders were not associated with perinatal outcomes, except for depression, which increased the risk of offspring admission to NICU, and anxiety disorders during pregnancy, which were associated with preterm birth and lower birth weight. Children born to mothers with depression/anxiety disorders before pregnancy, compared to children of mothers without these disorders, had an increased odds of a visit to ED for any reason (odds ratio (ORadj) = 1.26, 95% confidence interval (CI): 1.02-1.54), of an ED visit resulting in hospitalization (ORadj = 1.75, 95%CI: 1.27-2.42), and of planned hospital admissions (ORadj = 1.55, 95%CI: 1.01-2.40). These associations with healthcare utilization were similar for mental disorders also during pregnancy. The association pattern of maternal sleep disorders with perinatal outcomes and child healthcare utilization resembled that of maternal depression and/or anxiety disorders with these outcomes. Conclusion: Antenatal maternal mental health is a potential risk factor for child-health outcomes and healthcare use. Early maternal mental health interventions may help to promote child health and reduce healthcare costs. What is Known: • Poor maternal mental health affects pregnancy outcomes and child health, and children of mothers with mental health conditions tend to have increased healtcare utilization. • Parents with poor mental health often face challenges in caring for their children and have less parenting self-efficacy, which could potentially lead to frequent medical consultations for minor health issues. What is New: • Maternal pre-pregnancy mental disorders were not associated with preterm birth, low birth weight, SGA, and congenital anomalies, except for depression, which increased the risk of offspring admission to NICU. Anxiety disorders during pregnancywere associated with lower birth weight and an increased odds of preterm birth. • Maternal depression and/or anxiety and sleep disorders, both before and during pregnancy, were associated with an increase in children's healthcare utilization between 6 and 18months of life.