||Categories of birth defects were defined on the
basis of the three-digit codes of the International Classification of Diseases, 8th
Revision (ICD-8), with minor modifications. All registered birth defects except
congenital hip dislocation were used. For most infants, only a single defect was reported,
and it was assumed to be an isolated defect. We combined anencephalus and spina bifida as
neural tube defects (NTD). Down syndrome was separated from other recognized syndromes.
Multiple defects were combined in a separate category. Altogether, there were 25
categories of birth defects, one of which contained multiple defects.