Case ascertainment for state-based birth defects registries: characteristics of unreported infants ascertained through birth certificates and their impact on registry statistics in New York state.

Cases in the New York State Congenital Malformations Registry are reported by hospitals and physicians. This study was undertaken to determine whether case finding should be expanded to include routine matching of Vital Records files to the registry in order to identify unreported children. Matching of children who were born in 1983-86 and who had a congenital malformation noted on their birth certificate yielded 2837 children who were not in the registry. The hospital of record was asked to submit a registry report if the child's medical record contained a congenital malformation. Medical records for 1267 (45%) of these children indicated that the child was normal, with no mention of a malformation. Medical records could not be located for 137. Registry reports were submitted for 1433, 67 of whom were subsequently found in the registry, leaving 1366 bona fide new cases. These new cases differ significantly from registry cases for a number of birth certificate variables and type of congenital malformation. The birth certificate cases were more likely than registry cases to have only one malformation and to have only a minor malformation. The 1366 new cases comprised 2.1% of all registry cases for 1983-86. Their addition increased the statewide prevalence of major malformations by 1.7% from 416.5 to 423.4 per 10 000 livebirths. Except for anencephaly, the prevalence of specific malformations was not altered measurably by the addition of these cases. Lengthy and continuous follow-up was required to obtain registry reports. The small number of cases found does not seem to justify the amount of resources that would be required to use birth certificates routinely to augment case finding in New York State.