Purpose <p>Children of mothers experiencing mental illness are known to exhibit elevated rates of developmental delays. However, there is little evidence-based research on the impact of perinatal psychiatric admissions. Here we aim to quantify the association between maternal mental illness that leads to admission, and the likelihood of child developmental delay. In addition, we aim to understand the association between the timing of admission, the maternal diagnosis and the developmental domain in which delays manifest.</p> Methods <p>Retrospective cohort study of data from well-child visits of 634,918 term-born children assessed between 2016 and 2022 at the ages of 1 to 36 months. Likelihood of failure in milestone attainment was modelled based on maternal psychiatric information available through the national registry, and adjusted for demographic and birth data from the Israeli national program of Maternal Child Health Clinics. Secondary analyses examined stratification by maternal diagnosis, timing of admission and developmental domain, as well as comparison to the association between the likelihood of child developmental delay and maternal high scores on the Edinburgh Postnatal Depression Scale.</p> Results <p>Maternal mental illness that leads to psychiatric admission was associated with an increased odds ratio for child developmental delay (OR ~ 1.7). Higher odds ratio was associated with psychotic disorders diagnoses, multiple admissions and admissions between birth and child assessment. Nonetheless, increased odds were suggested also when first admission occurred after child assessment. In addition, increased odds were more evident when children were assessed at an older age.</p> Conclusions <p>Children of mothers experiencing mental illness exhibit higher rates of developmental delays, especially when mothers are admitted. Identifying and supporting high-risk dyads during the peripartum period is therefore critical for both mother and child.</p>