The project will generate early evidence about the impact of accountable care organization (ACO) implementation on maternal health outcomes, postpartum health care utilization, and maternal health equity. The research team will use the Massachusetts All-Payer Claims Database. The sample will be women ages 18–44 living in Massachusetts, with Medicaid or commercial insurance, with a delivery between 2015 and 2019. The key outcome measures will be: a binary measure of severe maternal morbidity at any time during delivery or the six months postpartum; and six binary measures of health care utilization in the six months postpartum: any hospital readmission, any emergency department visit, comprehensive postpartum visit, any other outpatient obstetric visit, any primary care visit, and any outpatient behavioral health visit. The team will then conduct four key analyses: 1) They will use an interrupted time-series approach to compare changes in maternal health outcomes for those who gave birth before and after Medicaid ACO implementation (March 1, 2018). 2) They will use a difference-in-difference analysis to compare changes in maternal health outcomes for those whose births were covered by Medicaid ACOs versus those with commercial insurance. 3) They will examine differences in outcomes across 17 Massachusetts Medicaid ACOs by ACO structure and ACO patient population. 4) They will conduct semistructured interviews with ACO administrators, clinicians, and postpartum women on facilitators/barriers in ACO design in addressing maternal health equity.