Ascertainment of birth defects: The effect on completeness of adding a new source of data

Background: The Western Australian (WA) Blah Defects Registry aims for complete ascertainment of birth defects in WA, but the proportions of birth defects in rural areas and in Aboriginal children are lower than in metropolitan and non-Aboriginal children. The effect on ascertainment of adding data from the Rural Paediatric Set-vice (RPS) was investigated. Method: A file of all cases of birth defects for children born 1980-1997 and recorded on the RPS database was linked to the Registry. Results: The addition of this new data source had little effect on the overall prevalence of birth defects (an increase from 5.38 to 5.41%). There was a slightly greater effect on the prevalence of birth defects in rural residents (4.67%-4.76%) and Aboriginal children (4.55-4.78%), although the prevalence for each of these groups is still less than for metropolitan residents and non-Aboriginal infants, respectively. All major categories of birth defects were represented in the new cases and, in general, their addition made little difference to the prevalence of each category. The exception was fetal alcohol syndrome, which increased from 0.13 per 1000 to 0.18 per 1000 once the 21 new cases from the RPS were added. Conclusion: Complete ascertainment of birth defects is important in developing and evaluating preventive programs, and in investigating clusters of birth defects.