What is Trisomy 9?

Most live born babies with this chromosomal disorder have mosaic trisomy 9

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Trisomy 9 is a rare and often fatal chromosomal abnormality that occurs in approximately 2.5 percent of spontaneous abortions—pregnancy losses that occur before the 20th week of pregnancy. 

Similar to trisomy 21 (also known as Down syndrome), trisomy 9 occurs when there are three copies of chromosome 9 present in a baby's cells, as opposed to two copies. Unlike trisomy 21, though, trisomy 9 is rarer, leads to more severe manifestations, and has a much lower rate of survival.

What are the Types of Trisomy 9?

There are three types of trisomy 9:

  • Full trisomy 9: All of the cells in the baby's body and placenta have three copies of chromosome 9.
  • Partial trisomy 9: There are two full copies of the ninth chromosome plus an additional partial copy.
  • Mosaic trisomy 9: The trisomy is present in some of the body's cells while other cells have a normal set of chromosomes. 

Full trisomy 9 is nearly always fatal, with the vast majority dying in the first trimester. While the majority of live born babies have mosaic trisomy 9, many die in infancy due to health problems caused by the disorder. That being said, there are some who survive beyond the first year of life.

Partial trisomy 9 may not affect the baby's life expectancy, but affected babies may have a range of common health and developmental problems.

What are the Signs or Symptoms of Trisomy 9?

The signs and symptoms present in trisomy 9 are variable.

Common findings on ultrasound include fetal heart defects and brain and spinal cord malformations.

Other potential signs and symptoms associated with trisomy 9 often include:

  • characteristic facial appearance (small head, broad nose with bulbous tip, cleft lip and/or palate, small jaw, low set ears, small eyes and/or eyelid folds that slant upwards 
  • vision problems
  • dislocated joints
  • underdeveloped genitalia
  • undescended tests in male babies
  • kidney cysts
  • feeding and breathing difficulties right after birth
  • failure to thrive
  • short stature
  • vision problems
  • variable cognitive disabilities and developmental delays

What Increases a Woman's Chance of Having a Baby with Trisomy 9?

Researchers have not identified any risk factors for trisomy 9, except there is a link with advanced maternal age, like trisomy 21. Otherwise, the condition appears to occur randomly. The only exception is if either parent has a condition known as balanced translocation affecting chromosome 9, which could increase the risk of having a baby with partial trisomy 9. However, partial trisomy 9 is relatively rare, compared to the other types.

How is Trisomy 9 Diagnosed?

Trisomy 9 may be diagnosed following a miscarriage. It is also possible to receive the diagnosis during pregnancy by either chorionic villus sampling (CVS) or amniocentesis. With placental tissue from a CVS or fetal cells from an amniocentesis, doctors can order a karyotype, which is a picture of the baby's chromosome. Sometimes the diagnosis is not made until a baby is born, and a karyotype is ordered for confirmation.

A Word From Verywell

It is scary and confusing to be told your baby has a genetic disorder, no matter how severe the condition may be. If you miscarry a baby with trisomy 9, you should know that the miscarriage was not your fault, and the odds are low that your next pregnancy will be affected. Sadly, trisomy disorders are just one of those things that happen sometimes. 

If you are currently pregnant and prenatal testing found trisomy 9, it's important to talk to an experienced genetic counselor or geneticist. There is no cure for trisomy 9, and treatment depends on the baby's unique signs and symptoms.

 

Sources:

Miryounesi M, Dianatpour M, Shadmani Z, Ghafouri-Fard S. Report of a case with trisomy 9 mosaicism. Iran J Med Sci. 2016 May;41(3):249-52.

NIH. Genetic and Rare Diseases Information Center. (2015). Mosaic trisomy 9.

Schwendenmann WD et al. Sonographic findings in trisomy 9. J Ultrasound Med. 2009 Jan;28(1):39-42.

Zen PR, Rosa RF, Rosa RC, Graziadio C, Paskulin GA. New report of two patients with mosaic trisomy 9 presenting unusual features and longer survival. Sao Paulo Med J. 2011 Dec;129(6):428-32.

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