Families who participate in the National Children’s Study will come from 105 previously selected sites, which include U.S. counties or groups of counties from across the country, and will include the different racial, ethnic, and socioeconomic groups. The Study will follow women who are pregnant as well as women who are not pregnant but may become pregnant during the Study's enrollment period. The subsequent children become the major focus of the Study after birth. A national probability method was used for site selection to ensure representation of the entire nation while preserving the local community dimensions of health and the environment. Criteria for site selection included demographics, number of births, and number of babies born with low birth weights. Please refer to the brochure The National Children’s Study and Your Community for more information.