Microdeletion syndromes are caused by chromosomal deletions that include several genes, but that are too small to be detected by conventional methods such as karyotyping. A microdeletion is identified by genomic position and by size, and many result in a clinically benign phenotype, however some are characterised by a complex disorder. Syndromes associated with microdeletions can present with a range of clinical features, including: intellectual disabilities, developmental delay, heart defects and failure to thrive.
The vast majority of microdeletions occur de novo i.e. they are not inherited from the parents. In some cases, however, they can be inherited from an apparently unaffected parent. Unlike common trisomies, microdeletions are not thought to be associated with advanced maternal age. Younger pregnant women statistically have a higher risk of a sub-microscopic aberration than for trisomy 21. This also must be considered in pregnancies resulting from egg donation.