Newborn birth defects (congenital anomalies) and chromosome abnormality:

Newborn birth defects or congenital anomalies, including delay in reaching development milestones and/or mental retardation, are relatively common and occur in about 3% of the general population. Although the cause often remains unknown in the vast majority of cases, in about 10% the cause has been found to involve an abnormality of the child’s chromosomes.

The types of chromosome abnormalities found can include extra or missing whole chromosomes (aneuploidy); e.g. Down syndrome is due to an extra whole chromosome 21, also known as Trisomy 21. Other chromosomal abnormalities or imbalances can involve an extra, or missing, part of a chromosome and is known as a chromosome duplication, or deletion. A chromosome test (karyotype) has previously been used to detect these types of abnormalities.

A limitation of conventional chromosome testing is that very small chromosomal imbalances, called microdeletions or microduplications, cannot be detected by routine microscope analysis. Some of these small chromosomal imbalances can be detected by a technique called fluorescent in-situ hybridisation (FISH) and are already known to cause Microdeletion syndromes. Alternatively, microarray comparative genomic hybridisation (CGH) is a new and more accurate test for detecting microdeletion and microduplication syndromes.