Impact of excluding cases with known chromosomal abnormalities on the prevalence of structural birth defects, Hawaii, 1986-1999.

Chromosomal abnormalities are more common in the presence of structural birth defects. However, much of the literature have only provided chromosomal abnormality rates for one or a few structural birth defects at a time. This study calculated the chromosomal abnormality rates for a number of structural birth defects using data from the Hawaii Birth Defects Program (HBDP) for deliveries during 1986-1999 and evaluated the impact of exclusion of cases with chromosomal abnormalities when calculating birth prevalence. The chromosomal abnormality rates were highest for endocardial cushion defect (40%) and omphalocele (27%), while no chromosomal abnormalities were reported for pyloric stenosis, persistent cloaca, and deficiency of lower limbs. The majority of chromosomal abnormality rates fell within a certain range, with 32 (63%) of the birth defect categories having chromosomal abnormality rates of 5-15%. The chromosomal abnormality rates also tended to be higher for multiple than for isolated cases. For three of the structural birth defects (ventricular septal defect, atrial septal defect, endocardial cushion defect), the birth prevalence of the defect, when cases with a chromosomal abnormality were excluded, was significantly lower than the birth prevalence that included those cases. Chromosomal abnormality rates varied by type of structural birth defect and presence of other major structural birth defects. For at least several structural birth defects, exclusion of cases with chromosomal abnormalities significantly underestimated the birth prevalence. This underestimation may be important, depending on the purpose of the analysis. Copyright 2004 Wiley-Liss, Inc.