Trisomy Birth Defects

Trisomy birth defects at a glance

  • Trisomy is a genetic defect involving an extra chromosome or part of a chromosome added to a normal pair (one from the mother and one from the father), resulting in a genetic abnormality and possible birth defects.
  • The most common trisomies are Trisomy 21 (also known as Down syndrome), Trisomy 18 and Trisomy 13.
  • Trisomies cause a wide range of problems, depending on the type, including miscarriage, stillbirths, catastrophic physical defects and mental retardation.

Types of trisomy birth defects

Trisomy can occur in sex chromosomes or non-sex chromosomes. Those occurring in sex chromosomes involve an extra X or Y chromosome added to the female’s normal XX pair or to the male’s normal XY pair. The resulting defects include reduced fertility and physical and behavioral differences that vary by individual and do not typically manifest themselves until puberty.

Trisomies of the non-sex chromosomes are referred to as autosomal trisomies, and can lead to miscarriage and stillbirth. Babies that do survive autosomal trisomy can experience a variety of birth defects, mental retardation and shortened life expectancy.

The most common autosomal trisomies are Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome) and Trisomy 13 (Patau syndrome). Trisomy 18 and Trisomy 13 are very rare but catastrophic for the child. Less than 10 percent of children born with these types of Trisomy survive to their first birthday.

Trisomy 21

Down syndrome is the one of the most common causes of birth defects. As women wait until later in life to have children, the number of Trisomy 21 births is increasing. Trisomy 21 births are five times higher in older mothers.

A child with Trisomy 21 may never fully develop physically and may have delayed social and reduced mental development. In addition to emotional issues, a child with Down syndrome may have the following medical conditions:

  • Heart defects
  • Gastrointestinal blockage
  • Sight and hearing problems
  • Thyroid problems
  • Other issues

Trisomy 18

This birth defect occurs in 1 out of every 6,000 births. About 50 percent of babies with Trisomy 18 are stillborn. Of the live births with Trisomy 18, half of the babies die within the first week, and only about 10 percent survive their first year. Problems include:

  • Congenital heart disease
  • Kidney problems
  • Lung infections
  • External and internal physical deformities
  • Cancers

Trisomy 13

Trisomy 13 occurs in 1 out of 10,000 births. More than 80 percent of those children die in their first year. Physical issues include:

  • Abnormal brain structure
  • Congenital heart disease
  • Rotation of internal organs
  • Difficulty breathing
  • Deafness
  • Seizures

Testing for Trisomy birth defects

One in 10 pregnant mothers have risk factors associated with Trisomy and are recommended to have genetic DNA testing. These risk factors include:

  • Being over age 35
  • A family history of a child with a genetic condition
  • An ultrasound indicating a genetic abnormality
  • Abnormal blood screens

Up until recently, the primary diagnostic fetal DNA testing options were amniocentesis, which involves drawing amniotic fluid from the abdomen, and chronic villus sampling, which is also an invasive procedure. Both carry a risk of ending the pregnancy.

A cell-free DNA diagnostic test involving a prenatal blood draw has recently become available. It has no risk to the pregnancy and is not invasive.

Though results of the cell-free DNA test are highly accurate, women who receive a positive test result are advised to undergo amniocentesis or chorionic villus sampling, which test for all 46 chromosomes. Current cell-free fetal DNA testing only detects eight chromosome problems, though the potential exists to detect issues in all 46 chromosomes.