What Is Trisomy 9?

Trisomy 9 is often fatal, ending in first-trimester miscarriage

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Sometimes cell division errors of the sperm or egg cells lead to a baby being conceived with a trisomy condition, or three copies of a particular chromosome rather than two. Depending on which chromosome is involved, the effects can range from mild to severe. Trisomy 9 usually falls on the severe end of the spectrum.

What Causes Trisomy 9

Trisomy 9 affects the ninth chromosome. 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 others have a normal set of chromosomes.

Full trisomy 9 is always fatal; most babies with full trisomy 9 are miscarried in the first trimester. Newborns with trisomy 9 often do not survive more than a few months, with most dying in the first week of life. Partial trisomy 9 and mosaic trisomy 9 have a more uncertain prognosis. Many babies with mosaic trisomy 9 die in infancy due to the health problems caused by the disorder, and those that survive usually have severe developmental impairments. Partial trisomy 9 may not affect the baby's life expectancy, but affected babies may have a range of common health and developmental problems.

Sometimes a phenomenon known as trisomy correction can occur very early in embryonic development, leading some cells to be unaffected by trisomy. Researchers believe this is what causes mosaic trisomy. No one is sure what causes the cell division errors or why some babies experience trisomy correction while others do not.

Researchers have not identified any risk factors for trisomy 9. 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.

Diagnosing Trisomy 9

Trisomy 9 may be diagnosed following a miscarriage, which could reveal that the miscarried baby had trisomy 9. It is also possible to receive the diagnosis during pregnancy by either chorionic villus sampling (CVS) or amniocentesis. Screening tests can show that a baby may be more likely to have a genetic disorder, but diagnoses can only be confirmed by genetic testing.

It is always scary and confusing to be told your baby has a genetic disorder, no matter how severe the condition may be. If you have miscarried 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 one of those things that just happen sometimes.

If you are currently pregnant and prenatal testing found trisomy 9, it's a good idea to talk to an experienced genetic counselor or geneticist. Because CVS tests cells from the placenta and an amniocentesis tests cells from the baby, doctors may recommend following CVS up with an amniocentesis. The prognosis is better if the amniocentesis fails to confirm the CVS findings; this means that the trisomy is most likely confined to the placenta. Unfortunately, prenatal testing currently cannot predict the outcome for individual babies.


Trisomy 9. TheFetus.net. Accessed: Mar 19, 2009. 

Trisomy 9 Mosaicism. University of British Columbia. Chromosomal Mosaicism. Accessed: Mar 19, 2009. 

Yeo, L. et al. "Prenatal Sonographic Findings Associated With Nonmosaic Trisomy 9 and Literature Review." Journal of Ultrasound in Medicine 2003. 22:425-430.

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