To investigate the association between prepregnancy obesity and birth outcomes using fixed effect models comparing siblings from the same mother.A total of 7496 births to 3990 mothers from the National Longitudinal Survey of Youth 1979 survey are examined. Outcomes include macrosomia, gestational length, incidence of low birthweight, preterm birth, large and small for gestational age (LGA, SGA), c-section, infant doctor visits, mother's and infant's days in hospital post-partum, whether the mother breastfed, and duration of breastfeeding. Association of outcomes with maternal pre-pregnancy obesity was examined using Ordinary Least Squares (OLS) regression to compare across mothers and fixed effects to compare within families.In fixed effect models we find no statistically significant association between most outcomes and prepregnancy obesity with the exception of LGA, SGA, low birth weight, and preterm birth. We find that prepregnancy obesity is associated with a with lower risk of low birthweight, SGA, and preterm birth but controlling for prepregnancy obesity, increases in GWG lead to increased risk of LGA.Contrary to previous studies, which have found that maternal obesity increases the risk of c-section, macrosomia, and LGA, while decreasing the probability of breastfeeding, our sibling comparison models reveal no such association. In fact, our results suggest a protective effect of obesity in that women who are obese prepregnancy have longer gestation lengths, and are less likely to give birth to a preterm or low birthweight infant.