Background: While maternal psychiatric illnesses are common, their effects on infant health outcomes are incompletely understood. We sought to describe the association between maternal psychiatric illnesses and infant prematurity, Emergency Department (ED) utilization, hospitalization, medical diagnosis, and death. Methods: We studied mothers who gave singleton livebirth in California from 2011 through 2017 and their infants, using the linked infant birth and death certificates, as well as maternal and infant ED and hospital discharge records. Maternal psychiatric illnesses were identified using International Classification of Diseases (ICD) codes from their ED and hospital discharge records during pregnancy, birth, and within 1 year of birth. We extracted infant health outcomes as premature birth (<37weeks gestation), ED visits and discharge diagnosis, hospitalization and discharge diagnosis, and death and cause of death. Log-linear regression was used to compare relative risk of infant health outcomes between those whose mothers did or did not have psychiatric diagnoses, adjusting for maternal age, maternal race/ethnicity, maternal education, payer for delivery, body mass index, adequacy of prenatal care, maternal non-psychiatric comorbidities, substance use, and parity. Results: Of 3,067,069 mother-infant pairs, 269,018 (8.8%) mothers carried at least one psychiatric diagnosis prior to, during, and/or after birth. Infants of mothers with a psychiatric diagnosis at any point in the perinatal period were more likely to be born prematurely, however, this was not statistically significant after adjusting for covariates. Infants of mothers with psychiatric illnesses were more likely to visit the ED (adjusted relative risk [aRR]: 1.2, 95% confidence interval [CI]: 1.2-1.2), be readmitted to the hospital after birth (aRR: 1.1, 95%CI: 1.1-1.1), or die (aRR: 1.7, 95% CI: 1.6-1.8) during the first year of life. These infants were also more likely to receive ED care or be hospitalized for non-specific symptoms (such as fussy, tired, apparent life-threatening event [ALTE], now referred to as brief resolved unexplained event [BRUE]), issues with feeding, accidental injuries and poisoning, and confirmed or suspected nonaccidental trauma (NAT), even after adjusting for covariates (table 1). Further, these infants were more likely to die from a birth defect/chromosomal abnormality (aRR: 1.3, 95%CI: 1.2-1.5), perinatal complications (aRR 1.7, 95%CI: 2.2-3.3), sudden infant death syndrome (SIDS) (aRR 2.7, 95%CI: 2.2- 3.3), and confirmed or suspected NAT (aRR: 2.9, 95%CI: 1.7-5.0). Increased risk of death from these causes remained statistically significant even when only mother-infant pairs with prenatal diagnosis of psychiatric illnesses were analyzed (table 2). Conclusion: This large cohort study shows the association between maternal peripartum psychiatric illness and increased infant healthcare utilization and adverse health outcomes. This study underscores the urgent need to better understand how maternal psychiatric illnesses confer the risk of adverse infant health outcomes and to devise effective interventions to mitigate this risk.