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Trisomy 9


Updated June 23, 2014

Humans are supposed to have 46 chromosomes, made up of 23 pairs. Sometimes errors in cell division 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 in the trisomy, the effects can range from mild to severe. Trisomy 9 is a condition that usually falls on the more severe end of the spectrum.

What Trisomy 9 Means:

Trisomy 9 is a trisomy affecting the ninth chromosome. Full trisomy 9 would mean that all of the cells in the baby's body and placenta had three copies of chromosome 9. It is also possible to have partial trisomy 9, meaning there are two full copies of the ninth chromosome plus an additional partial copy. A third type of trisomy 9 involves mosaicism, meaning that the trisomy is present in some of the body's cells but other cells have a normal set of chromosomes.

Prognosis of Trisomy 9:

Full trisomy 9 is always fatal; most babies with full trisomy 9 are miscarried in the first trimester. Those that make it to birth typically will 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.


The cause of trisomy 9 is an error in cell division affecting either the sperm or egg, meaning that the problem is already present at conception. 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 the origin of mosaic trisomy disorders. No one is sure what causes the cell division errors and why some babies experience trisomy correction whereas others do not.

Risk Factors:

Researchers have not identified any risk factors for having a baby affected by 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, but partial trisomy 9 is relatively rare compared to other types.


Diagnosis of trisomy 9 could come in the form of followup testing after a miscarriage, which could reveal that the miscarried baby had trisomy 9. It is also possible to receive the diagnosis during pregnancy by either 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.

If Your Baby Has Been Diagnosed:

It is always scary and confusing to be told your baby has a genetic disorder of any kind, no matter where that disorder falls on the spectrum for severity. If you have miscarried a baby who was found to have 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 have had a CVS or amniocentesis test that found trisomy 9, it's a good idea to talk to an experienced genetic counselor or geneticist. Given that most babies with full trisomy 9 do not make it past the first trimester, a baby that survives to the point of having a CVS or amniocentesis is most likely to have a partial or mosaic trisomy 9. Because a CVS tests cells from the placenta and an amniocentesis tests cells from the baby, doctors may recommend following up with an amniocentesis in cases that the CVS shows a trisomy condition. The prognosis is better if the amniocentesis fails to confirm the CVS findings; this would mean that the trisomy is likely to be confined to the placenta. Babies with mosaicism for trisomy 9 that is confirmed in an amniocentesis can be anywhere from mildly to severely affected. Unfortunately, prenatal testing currently cannot predict the outcome for individual babies.


Trisomy 9. TheFetus.net. Accessed: Mar 19, 2009. http://www.thefetus.net/page.php?id=917.

Trisomy 9 Mosaicism. University of British Columbia. Chromosomal Mosaicism. Accessed: Mar 19, 2009. http://www.medgen.ubc.ca/robinsonlab/mosaic/specific/trisomy9.htm.

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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