Your guide to health and prenatal genetics
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What is a trisomy?

Trisomy (means “three bodies”) is a chromosomal disorder characterised by an additional chromosome, when there are three rather than two of a particular chromosome, thus bringing the total chromosomes number to 47 (instead of normal 46 number). A trisomy is a type of aneuploidy. It can occur with any chromosome, but often result in spontaneous miscarriage (abortion), since the conditions are not compatible with life. In fact, trisomies represent around 40% of spontaneous abortions and stillbirths. However, some fetuses with trisomy of particular chromosomes can survive till birth. Children affected by trisomy usually have a range of birth defects, delayed development, intellectual disabilities and shortened life expectancy.

With few exceptions, trisomies do not appear to be compatible with life and represent about 35% of spontaneous abortions.

Most common trisomies in embryos, who survive till birth are:

  • trisomy 21 (Down syndrome)
  • trisomy 18 (Edwards syndrome)
  • trisomy 13 (Patau syndrome)
  • trisomy 9
  • trisomy 8
  • trisomy 22

The first three are the commonest trisomies in human.

The risk of most trisomies of autosomal chromosomes is highly dependent on maternal age because of increased errors in cell divisions with woman’s age. The risk increases sharply exponentially when women approach the end of their reproductive life span.

Down syndrome (T21) is a relatively common birth defect, on average affecting about 1 in every 750 births and the risk is related to maternal and gestational age. Even though younger mothers have lower risk, their birth rate is much higher, and therefore more than half of the mothers of all Down syndrome babies are younger than 35 years.

For more specific risk estimation and available NIPT choices please refer to our educational MyAdvisor Tool.

Trisomy of sex chromosomes can include XXX (triple X syndrome), XXY (Klinefelter syndrome), XYY. Trisomy of the sex chromosomes normally has less severe consequences than trisomy of the autosomal chromosomes and their risk is not generally dependent on maternal age (except XXX and XXY, which have significant age effect).