Objectives. To examine the effect of the January 2017 leak of the federal government's intent to broaden the public charge rule (making participation in some public programs a barrier to citizenship) on immigrant mothers and newborns in New York State. Methods. We used New York State Medicaid data (2014-2019) to measure the effects of the rule leak (January 2017) on Medicaid enrollment, health care utilization, and severe maternal morbidity among women who joined Medicaid during their pregnancies and on the birth weight of their newborns. We repeated our analyses using simulated measures of citizenship status. Results. We observed an immediate statewide delay in prenatal Medicaid enrollment by immigrant mothers (odds ratio = 1.49). Using predicted citizenship, we observed significantly larger declines in birth weight (-56 grams) among infants of immigrant mothers. Conclusions. Leak of the public charge rule was associated with a significant delay in prenatal Medicaid enrollment among immigrant women and a significant decrease in birth weight among their newborns. Local public health officials should consider expanding health access and outreach programs to immigrant communities during times of pervasive antiimmigrant sentiment. (Am J Public Health. 2022; 112(12):1747-1756. https://doi.org/10.2105/AJPH.2022.307066).