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 The test is good in terms of determining whether or not the mother has a high chance of carrying a child with a genetic abnormality - however, doctors still require an invasive test such as an amniocentesis to clearly visualize the fetal chromosomes and rule out trisomies.

Fetal trisomies are extra copies of chromosomes in the human cell.  Humans typically have 23 pairs of chromosomes, but occasionally an extra copy will occur, leading to certain birth defects such as Down syndrome, Edwards syndrome or Patau syndrome.  

Down syndrome, which is caused by an extra copy of chromosome 21, is associated with intellectual disabilities, digestive issues and congenital heart defects.  It occurs in one out of every 740 newborns.

An extra copy of chromosome 18, on the other hand, causes Edwards syndrome - and in some cases, miscarriage.  Babies born with Edwards syndrome usually have congenital heart defects and various other medical conditions, including shortened lives.  It occurs in one out of every 5,000 newborns.

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