Trisomy 9 is a rare and often fatal chromosomalabnormality. This condition occurs in approximately 2.4% of pregnancy losses that happen before the 20th week of pregnancy (also called spontaneous abortions).
Similar to trisomy 21 (also known as Down syndrome), trisomy 9 occurs when there are three copies (as opposed to the usual two) of chromosome 9 present in a fetus's cells. Trisomy 9 is rarer than trisomy 21 and has more severe manifestations. It also has a much lower survival rate. Learn more about trisomy 9, including symptoms, diagnosis, and life expectancy.
Types of Trisomy 9
There are three types of trisomy 9. Each type has its own genetic characteristics and physical manifestations.
- 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: 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 of affected fetuses dying in the first trimester. The majority of live-born babies have mosaic trisomy 9. Many will die in infancy from health problems caused by the disorder.
However, some infants with mosaic trisomy 9 dosurvive beyond the first year of life.
Partial trisomy 9 does not always affect an infant's life expectancy. However, babies with the condition do have a range of health and developmental problems.
Signs and Symptoms of Trisomy 9
The signs and symptoms of trisomy 9 are variable. Some can be detected before a baby is born while others may not be apparent until after birth.
Prenatally, common findings on ultrasound include fetal heart defects and brain and spinal cord malformations. After a baby is born, signs and symptoms associated with trisomy 9 include:
- Characteristic facial appearance (small head, broad nose with a 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 testes in male infants
- Kidney cysts
- Feeding and breathing difficulties at birth
- Failure to thrive
- Short stature
- Vision problems
- Variable cognitive disabilities and developmental delays
Risk Factors for Trisomy 9
In most cases, trisomy 9 appears to occur randomly rather than as a result of specific, identified risk factors. The exception is if either parent has a condition called balanced translocation affecting chromosome 9, as research has found an association between the condition and an increased risk of having a baby with partial trisomy 9.
However, compared to the other types, partial trisomy 9 is relatively rare.
Diagnosis of Trisomy 9
Trisomy 9 is frequently diagnosed following a miscarriage, but it can also be detected during pregnancy by chorionic villus sampling (CVS) or amniocentesis. Using placental tissue from CVS or fetal cells from amniocentesis, a doctor can order a picture of the baby's chromosome (karyotype). If indications of trisomy 9 are seen at birth, a karyotype can be completed to confirm the diagnosis.
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 have a miscarriage, know that the loss was not your fault and the odds are low that your next pregnancy will be affected.
If you are currently pregnant and prenatal testing detects trisomy 9, you will want to speak to an experienced genetic counselor or geneticist. While there is no cure for the condition, understanding which form of trisomy 9 has been diagnosed will help you make decisions about your pregnancy; labor and delivery, and postpartum care and treatment for your baby.
Genetic Disorders in Pregnancy
7 Sources
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National Organization for Rare Disorders (NORD). Mosaic Trisomy 9. NORD Rare Disease Database.
National Center for Advancing Translational Sciences (NCATS). Mosaic Trisomy 9. Genetic And Rare Diseases Information Center (GARD).
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;129(6):428-32. doi:10.1590/S1516-31802011000600010+
Tiong, K., Cotterill, A. & Falhammar, H. Adult case of partial trisomy 9q.BMC Med Genet11,26 (2010). doi:10.1186/1471-2350-11-26
National Organization for Rare Disorders (NORD). Trisomy 9p (Multiple Variants). NORD Rare Disease Database.
Lee CY, Su HJ, Cheng YT, Ku YL, Ngo YG, Chen CM, et al. Detection of fetal trisomy 9 mosaicism by noninvasive prenatal testing through maternal plasma DNA sequencing.Taiwan J Obstet Gynecol. 2018;57(4):594–597. doi:10.1016/j.tjog.2018.06.021
Additional Reading
9Tips: Trisomy 9 International Parent Support. trisomy9.org.
By Krissi Danielsson
Krissi Danielsson, MD is a doctor of family medicine and an advocate for those who have experienced miscarriage.
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