BACKGROUND: There is currently no population-based research on the maternal characteristics or birth outcomes of U.S. women with intellectual and developmental disabilities (IDDs). Findings from small-sample studies among non-U.S. women indicate that women with IDDs and their infants are at higher risk of adverse health outcomes. PURPOSE: To describe the maternal characteristics and outcomes among deliveries to women with IDDs and compare them to women with diabetes and the general obstetric population. METHODS: Data from the 1998-2010 Massachusetts Pregnancy to Early Life Longitudinal database were analyzed between November 2013 and May 2014 to identify in-state deliveries to Massachusetts women with IDDs. RESULTS: Of the 916,032 deliveries in Massachusetts between 1998 and 2009, 703 ( < 0.1%) were to women with IDDs. Deliveries to women with IDDs were to those who were younger, less educated, more likely to be black and Hispanic, and less likely to be married. They were less likely to identify the father on the infant's birth certificate, more likely to smoke during pregnancy, and less likely to receive prenatal care during the first trimester compared to deliveries to women in the control groups (p < 0.01). Deliveries to women with IDDs were associated with an increased risk of adverse outcomes, including preterm delivery, very low and low birth weight babies, and low Apgar scores. CONCLUSIONS: Women with IDDs are at a heightened risk for adverse pregnancy outcomes. These findings highlight the need for a systematic investigation of the pregnancy-related risks, complications, costs, and outcomes of women with IDDs. Inc.