Child overweight and obesity in the U.S. is a significant public health issue. In 2008 nearly one-third of all U.S. children ages two to seventeen were obese or overweight [11]. For young children ages two to five, fully 21.2% were overweight (at or above the 85th%ile based on the CDCﾒs sex-specific BMI-for-age growth charts), while 10.4% of preschoolers were obese (at or above the 95th%ile) [11]. The prevalence of overweight and obesity among U.S. children has implications for children's future health and the health trajectory of the nation. For example, children who are overweight are more likely to grow up to be overweight or obese [1-3], to suffer health consequences both as children and later in life [4-6], and to experience social and behavioral difficulties [7,8]. Moreover, in 2009 the estimated annual healthcare costs in the U.S. related to obesity topped $145 billion [9], a figure which is expected to increase as obese children age and develop other health problems [10]. Thus, while recent data show that trends in childrenﾒs overweight and obesity rates are stabilizing, obesity continues to be a substantial problem, including among younger preschool-aged children, and identifying the contributing factors to it an important goal. By and large, scientists have identified nutrition and physical activity as the primary determinants of weight status for children [12]. Yet social factors have been shown to play an important role too. In examining this side of childrenﾒs weight development, parentsﾒ socioeconomic status has emerged as a primary social predictor. In particular, obesity in the U.S. is more prevalent among children who are racial or ethnic minorities [11,13], and whose parents have less income and lower levels of education [13]. Differences in parenting styles [14], culture [15], exposure to stressors [16,17], and neighborhood context [18] have been presented as some of the main mechanisms connecting parentsﾒ socioeconomic status with childrenﾒs risk of obesity. Going beyond this well-developed area of research, however, another social factor and indicator of family socioeconomic background that may be associated with childrenﾒs risk of obesity is family structure. Increasing family complexity over the past three decades in the United States means that more children are growing up in homes without two biological parents. Yet few studies have considered the role of different family structures in childrenﾒs weight status, and among those that have, even fewer have constructed and assessed categories for family structure that represent the diversity among U.S. families today.