Hypoplastic left heart syndrome and other left heart disease: evolution of understanding from population-based analysis to molecular biology and back again--a brief overview.

Congenital cardiac disease is the most common birth defect, occurring in approximately 1 in 1000 live births. Congenital cardiac defects have associations, whether with gender, race, or specific chromosomal abnormalities, potentially allowing grouping of defects to be studied in an effort to develop an understanding of aetiological factors. The Baltimore-Washington Infant Study provides full ascertainment of a population of infants with congenital cardiac disease born in a defined geographic region. The fundamental hypotheses generated at the inception of the Baltimore-Washington Infant Study included the central idea that the outcome of birth, including the development of congenital cardiac malformations, was influenced by environmental factors and their route of introduction into a genetically susceptible host. Evidence exists that supports the concept that both genetic and environmental factors contribute to the development of diseases of the left heart.