Consanguinity, the marriage between relatives, has been associated with adverse child health outcomes because it increases homozygosity of recessive alleles. The objective of this study was to assess the effect of consanguinity on the birth weight of newborns in Greater Beirut, Lebanon. Cross-sectional data were collected on 10,289 consecutive liveborn singleton newborns admitted to eight hospitals belonging to the National Collaborative Perinatal Neonatal Network during the years 2000 and 2001. Birth weight was modeled by use of the fetal growth ratio, defined as the ratio of the observed birth weight to the median birth weight for gestational age. A mixed-effect multiple linear regression model was used to predict the net effect of first- and second-cousin marriage on the birth weight for gestational age, accounting for within-hospital clustering of data. After controlling for medical and sociodemographic covariates, the authors found a statistically significant negative association between consanguinity and birth weight at each gestational age. No significant difference was observed in the decrease in birth weight between the first- and second-cousin marriages. Overall, consanguinity was associated with a decrease in birth weight for gestational age by 1.8% (beta = -0.018, 95% confidence interval: -0.027, -0.008). The largest effects on fetal growth were seen with lower parity and smoking during pregnancy.