**3.1.1 Chromosome defect**

Defects in chromosomes associated with CHD are diverse; some examples are aneuploidy or polyploidy, improper rearrangement during mitosis and meiosis, translocation, inversion or deletions. Importantly, certain chromosomes were reported to have a greater degree of significance and of percentages to heart development, and thus the same defects in different chromosomes may not result in similar defects (Table 4). About 0.30-2.0% of all live births have chromosomal defects, usually the chromosomal defects were aneuploidy and trisomy 21, 18, 13 (Dolk et al., 2010). Among all CHDs detected during infant period, the chromosomal defects account for approximately 6 - 10% (Ferencz et al., 1989; Tennstedt et al., 1999; Zhang et al., 2010). In Table 4, defects in chromosomes X, 3, 4, 5, 7, 8, 9, 10, 11, 13, 17, 18, 21 and 22 showed association with CHD.

Nonetheless, the table summarizes the data reported by different studies, some conducted in different times and places. The incidence of CHD generally depends on multiple factors besides the type of genetic disorders and the chromosome where the disorders take place. The other factors include how many fetuses are conceived by the mothers, and how many of these fetuses reach term alive. Further, the affected number of fetuses also depends on the rate of the survival of the affected fetuses and the increased use of therapeutic abortion.
