Physical size at birth (primarily birth weight) is a key predictor of infant mortality and morbidity and may serve as a predictor of risk to chronic disease in adulthood. By use of birth records, it is sometimes possible to accumulate large, even massive, data sets that can permit analyses to separate genetic and environmental factors controlling variation in the complex phenotype, birth weight. Two studies of birth records, one from Norway and one from Lebanon, demonstrate how hospital- or registry-based data sets can be used to address fundamental questions about genetic control. The Medical Birth Registry of Norway has birth certificate data dating back to 1967 and allowed nuclear families to be reconstructed by linking children and their parents. Path models were used to estimate heritability of head circumference, along with birth weight and length for over 100,000 reconstructed families. A hospital-based study of birth records in Beirut, Lebanon, collected information on over 10,000 births, including sufficient numbers of marriages between first and second cousins to estimate inbreeding effects. Both of these studies confirm that birth weight is not simply due to the direct effects of the baby's genes but is a complex phenotype reflecting the effects of maternal genes and environments.