Your guide to health and prenatal genetics
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Assess your personalised prenatal risks, choose screening tests, and find nearby maternity clinics

What is an aneuploidy?

Aneuploidy is an abnormal number of chromosomes, with usually one extra (a trisomy) or missing (monosomy) copy. Since each chromosome contains hundreds of genes, addition or loss of even a single chromosome disrupts the existing delicate balance of gene products in cells, and in most cases, is not compatible with life or causes impaired development.

Aneuploidy (trisomy or monosomy) is the most commonly identified chromosome abnormality in humans, occurring in at least 10% of human pregnancies and, for women nearing the end of their reproductive lifespan, the incidence may exceed 50%.

Most aneuploid fetuses cease to develop in utero (a womb), which makes this the leading genetic cause of pregnancy loss. However, some aneuploid fetuses survive to the birth term and, generally, an aneuploidy is the most common known cause of human mental retardation.

There are aneuploidies of autosomal and sex chromosomes. Sex chromosomal aneuploidies occur in one out of 500 male births and one out of 850 female births.