A trisomy is a type of aneuploidy in which there are three chromosomes instead of the usual pair. Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome) and Trisomy 13 (Patau syndrome) are the three most commonly occuring autosomal chromosome aneuploidies in live births. These chromosomal conditions are caused by the presence of an extra copy or partial copy of chromosome 21, 18 or 13 respectively. This additional genetic material can cause dysmorphic features, congenital malformation and dierent degrees of intellectual disability.
2. Deletion Syndromes
Deletion syndromes are defined as a group of clinically recognisable disorders characterised by a small deletion of a chromosomal segment. The size and position of the deletion determine which clinical features are manifested and how severe they are. Clinical features of deletions can include developmental delays and intellectual disability, growth dierences, behavioural problems, feeding diculties, low muscle tone, seizures, dysmorphic features and a pattern of varying malformations.
3. Sex Chromosomal Aneuploidies
Sex chromosome aneuploidy is defined as a numeric abnormality of an X or Y chromosome, with addition or loss of an entire X or Y chromosome. Although most cases of sex chromosome aneuploidies are generally mild without intellectual disability, some have a well-established phenotype that can include physical abnormalities, learning delays and infertility.
ISPD( International Society of Prenatal Diagnosis) recognises that NIPT can be helpful as a screening test for women who are at high risk of Trisomy 21 with suitable genetic counselling. A positive test should be con?rmed through invasive testing.
The NSGC(National Society of GeneticCounselors) supports NIPT as an option for patients whose pregnancies are considered to be at an increased risk of certain chromosome abnormalities. Patients whose NIPT results are abnormal, or who have other factors suggestive of a chromosome abnormality, should receive genetic counselling and be given the option of standard confirmatory diagnostic testing.