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Objective This study aimed to estimate the prevalence of measles immunity in a cohort of pregnant women in New York City and determine if there is a positive correlation of measles immunity with patient demographics, rubella immunity, number of measles, mumps, and rubella vaccine (MMR) doses received, and age at last vaccination. Study Design This is a cross-sectional study of pregnant patients seen at a single institution from January 2019 to May 2019. Patients were classified as measles and rubella immune or nonimmune using commercial immunoglobulin G (IgG) tests. Patient characteristics were compared using t-tests, Chi-square tests, or Fisher's exact tests as appropriate. The association of age at last vaccination with immunity status was assessed using multivariable logistic regression adjusted for age at presentation. The utility of rubella IgG for distinguishing measles immunity was assessed using receiver operating characteristic curve analysis. Results Serologic immunity for measles and rubella was obtained for 1,366 patients. Of these, 1,047 (77%) were measles immune and 1,291 (95%) were rubella immune. Patients born after 1989 were less likely to be immune to measles, while multiparity and private insurance were associated with increased measles immunity. Documentation of MMR vaccination was available for 140 (10%) patients. Of these, 44 (31%) were serologically nonimmune to measles and 9 (6.4%) were nonimmune to rubella. In patients known to have received one dose of MMR, 62% (24/39) were immune to measles with an improvement to 72% (69/96) among those who received two or more doses. Age at last vaccination was not associated with measles immunity. Rubella IgG level was a poor predictor of positive measles titer (area under the curve = 0.59). Conclusion Approximately one of every four pregnant patients is serologically measles nonimmune, even among women with documented MMR vaccination or documented rubella immunity. These findings raise concerns that relying on vaccination history or rubella immune status may not be sufficient to assure protection from infection with measles. If further suggests that measles serology should be added to routine prenatal laboratory testing to identify nonimmune patients that may benefit from postpartum vaccination. Key Points