Chromosome Disorders: What is Trisomy?

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March 29, 2017


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The human body is made from 23 pairs of chromosomes for a total of 46. Chromosomes are made up of small packets of genes that include DNA. “Trisomy” is a genetic disorder in which a person has three copies of a chromosome instead of two.

Having an extra chromosome changes the way a baby develops physically and intellectually. These changes can present a baby and the subsequent adult with a number of mental and physical challenges.

Trisomy can occur with any chromosome, but there are mostly three well-known syndromes that develop as a result of the duplication.

The most common one is Trisomy 21, more widely known as Down syndrome. Down syndrome occurs in about 1 in 691 live births in the United States, according to the U.S. Centers for Disease Control and Prevention (CDC). People with Down syndrome usually have mild-to-moderate intellectual and developmental disabilities. They also have some distinct physical features, such as a flat-looking face, and they are at risk for a number of other health conditions, including hearing and vision loss.

Trisomy 18, also known as Edwards syndrome, takes place 1 in about 5,000 births. These babies have more serious intellectual and developmental disabilities, including life-threatening issues that are present at birth, such as heart defects and other malformed organs. Edwards syndrome has a higher occurrence among black and non-Hispanic populations, according to the CDC.

Trisomy 13 takes place in approximately 1 in 7,900 births in the U.S. This chromosomal abnormality is called Patau syndrome. This trisomy condition has the most severe intellectual disabilities, serious birth defects and associated health conditions, such as heart problems, and brain and spine abnormalities. Babies with trisomy 13 often have birth defects that affect many parts of the body, including extra fingers or toes, cleft lip or weak muscle tone. Only five to 10 percent of children with Trisomy 18 or Trisomy 13 live past their first year, says the National Institutes of Health.

“The United States has a slightly higher rate of babies born with trisomy syndromes than those in other developed countries because a lot of women here are delaying having children,” said Anthony Lai, M.D., an obstetrician and gynecologist, and maternal and fetal medicine specialist, affiliated with South Miami Hospital, Baptist Hospital and West Kendall Baptist Hospital.

Giving birth at age 35 or older is one of the main risk factors of having a baby with a chromosomal abnormality. Having a previous child with a trisomy condition or a close family member also increases the risk, Dr. Lai says.

Testing for Chromosomal Abnormalities

“Screening for chromosomal abnormalities is part of routine discussions during regular maternity visits,” said Dr. Lai.

The standard screening is a blood test done between 11 and 14 weeks of pregnancy. It’s done to determine if a mother is at low or high risk of having a baby with a trisomy condition.

If the screening test results are positive, indicating a higher chance of abnormality, a diagnostic test is then performed. Chorionic villus sampling (CVS) tests material from the placenta. And amniocentesis is a test that examines amniotic fluid, which is the fluid from the sac that surrounds the baby. These diagnostic tests are performed around 15 to 17 weeks of pregnancy.

There’s also a relatively new test that’s DNA-specific. It looks at the mother’s blood to detect small pieces of the developing baby’s DNA and any chromosomal changes in it. This more advanced test is usually recommended for mothers who are at high-risk of having a baby with a chromosomal abnormality.

Because no one knows why these chromosomes develop abnormally, there’s no way to prevent the conditions they cause.

“There’s no real way of reducing risk other than conceiving through infertility methods since the embryo can be tested before implantation,” Dr. Lai said. “Knowing the results of the tests help us as physicians better prepare the mother for having a child with developmental conditions so she has a reasonable idea of what to expect.”

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