How is a Trisomy Disorder Diagnosed?

A trisomy chromosome disorder occurs when an individual has one additional chromosome of DNA, resulting in 47, instead of 46, chromosomes. These conditions can be diagnosed either before or after birth through a variety of techniques.

Prenatal Screening

Prenatal screening tests will provide your doctor with information that allows him or her to understand the risk that your baby will be born with a trisomy disorder. These are not diagnostic techniques. Prenatal screenings are used to monitor the health of your baby and to monitor for signs of chromosomal disorders.

The types of prenatal screening include:

Ultrasound – Ultrasound scans are used to track the growth and development of your baby. This imaging technique can show physical abnormalities that may be associated with trisomy disorders.

Triple/quad screening – During pregnancy, specific hormones and proteins that are produced by the placenta and the fetus circulate in the mother’s bloodstream. In this test, either three or four substances are analyzed in the mother’s blood to determine the risk that a trisomy disorder is present: alpha-fetoprotein, human chorionic gonadotropin, estriol, and, in the case of quad screening, inhibin-A.

Noninvasive prenatal genetic testing – Noninvasive prenatal genetic testing analyzes cell-free DNA from the placenta that circulates in the mother’s bloodstream to assess the risk that the baby may have a trisomy disorder. This screening test can be performed as early as week 10 in pregnancy and can indicate to your doctor if further testing may be necessary.

Some common signs of trisomy disorders before birth include:

  • Additional amniotic fluid around the baby
  • Physical defects associated with trisomy disorders, such as heart abnormalities, which may be picked up during an ultrasound scan
  • The baby being small in size for its gestational age
  • The placenta being smaller than what it should be
  • The baby is less active than expected

Prenatal Diagnostic Testing

If the results from your prenatal screening test indicate that your baby is at a high risk of having a trisomy disorder, you will be presented with the option of diagnostic testing. Diagnostic testing will provide your doctor with a diagnosis, or a definitive answer as to whether your child has a trisomy disorder or not.

Diagnostic testing options may include:

Amniocentesis – Amniocentesis involves collecting a sample of the amniotic fluid and analyzing it to see if there is an extra chromosome.

Chorionic villus sampling – For this test, a sample of cells called the chorionic villi, a part of the placenta, is analyzed for an extra chromosome.

Diagnosing Trisomy Disorders after Birth

Some trisomy disorders, such as Down syndrome, can be diagnosed after the birth of your child by looking at his or her physical characteristics. If the baby is suspected to have a trisomy disorder, a blood test will be performed to analyze the chromosomes of the baby and to confirm the disorder.

Where Can You Find More Information?

If you would like to learn more about trisomy disorders and how they are diagnosed, a genetic counselor can help answer your questions. These medical professionals are specifically trained in the evaluation of a couple’s personal and family histories to analyze the risk that they or their child may develop an inherited condition.



Image courtesy of [Praisaeng] at

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