Substance use disorder (SUD) during pregnancy increases risks of adverse outcomes for mothers and children. Because Medicaid covers about half of all births and maternal SUD is a costly problem, describing the timing of enrollment and health care that Medicaid-enrolled pregnant women with SUDs receive is critical to understanding gaps in the timeliness and specificity of SUD diagnosis and treatment for pregnant women with SUDs. We used linked maternal and infant Medicaid claims and enrollment data and infant birth records from three states (n=72,086 mother-infant dyads) to estimate the share of sample women diagnosed with a specified SUD (e.g., opioid use disorder) before or during the birth month, with a specified SUD after the birth month, and with only an unspecified SUD diagnosed (e.g., drug use disorder complicating pregnancy). We also examined the timing of first observed Medicaid enrollment, SUD diagnosis and treatment, and maternal and infant costs. In the 24 months surrounding birth, 3.6% of women had a specified SUD diagnosis first observed before or during the birth month, 1.7% had a specified SUD diagnosis first observed after the birth month, and 6.0% had an SUD diagnosis that was not specified. Most women with a specified SUD diagnosis were enrolled in Medicaid before or early in pregnancy and initiated prenatal care in the first or second trimester, yet nearly one-third of these women received their specified SUD diagnosis after the birth month. Less than two-thirds of women with a specified SUD diagnosis received any SUD treatment during the study period (59.9% among those identified before or during the birth month and 63.1% among those observed after the birth month), and women with an unspecified SUD were about half as likely to get treatment (28.6%). Among treated women, more than two-thirds had the first observed treatment in the same month as their first observed SUD diagnosis. Findings point to a critical need for interventions as well as substantial opportunities to improve the identification of substance use-related needs and provision of treatment among women who birth in Medicaid. Changes in Medicaid and other public policy to reduce disincentives for pregnant and parenting women to report substance use during medical visits and to increase providers' abilities and motivation to equitably screen for as well as treat women with SUDs before, during, and after pregnancy could improve outcomes for mothers and their children. Improvements in SUD diagnosis would also improve prevalence estimates of specific types of SUD, which could contribute to better Medicaid policies aimed at prevention and treatment.