The unique identifiers of those women eligible for dental services during pregnancy were sent to the birth certificate data manager, who then selected the case-control study population. The case group consisted of all pregnancies resulting in low-birth-weight infants. Four normal birth weight infants (2,500–5,414 g) were randomly selected for each case without matching. Both the Washington Dental Service and birth certificate data managers stripped the data files of personal identifiers and sent them to the University of Washington, where linking occurred using the patient unique identifier. Human subject approval was obtained from the Human Research Review Board of the Washington State Department of Health.