Trisomy 9 is a rare but lethal chromosomal disorder that occurs when chromosome 9 appears three times (trisomy) in fetal cells instead of the usual two times. Only about 0.1% of trisomy 9 pregnancies result in a live birth with survival periods ranging from minutes to 9 months. Most trisomy 9 pregnancies end in early miscarriage (spontaneous abortions). About 2.4% of miscarriages occur before the 20th week of pregnancy.
Those born with trisomy 9 often have several abnormalities of the craniofacial (skull and face) region, brain, heart, kidneys, and musculoskeletal system as well as nervous system problems, neurodevelopmental delay, and growth deficiencies.
If trisomy 9 is detected during prenatal testing, it is necessary to speak to an experienced genetic counselor or geneticist. There is no cure for the condition. Understanding the type of trisomy 9 that has been diagnosed can help parents when it comes to making decisions about pregnancy, labor and delivery, postpartum care, and treatment for the baby.
What are the types of trisomy 9?
Each type of trisomy 9 has its own distinct genetic features and physical manifestations:
- Full trisomy 9: All cells of the fetus and placenta have three copies of chromosome 9. Full trisomy 9 is almost always fatal, with most of the affected fetuses dying in the first trimester, causing miscarriage.
- Mosaic trisomy 9: Trisomy 9 is present in only some of the cells of the fetus, while other cells have the normal set of chromosomes. Most babies who have mosaic trisomy 9 are born alive. However, many die in infancy due to health complications. Rarely, some infants with mosaic trisomy 9 live beyond the first year of life.
- Partial trisomy 9 (trisomy 9p): Two full copies of chromosome 9 and a part of an additional third partial copy usually of the short arm of the chromosome (arm p) is present in the fetal cells. Trisomy 9p is usually one of the most frequent autosomal anomalies that does not always affect the life expectancy of an infant. However, babies with this anomaly have a range of health problems and developmental delays.
What causes trisomy 9?
Most cases of trisomy 9 occur because of random errors during chromosomal separation called nondisjunction, causing some eggs or sperm to have an abnormal number of chromosomes. If an egg or sperm with an extra chromosome 9 contributes to the genetic makeup of the embryo, there will be an extra copy of chromosome 9 in each cell.
In other cases, the egg and sperm may have a normal number of chromosomes, but an error may occur during cellular division after fertilization.
What are risk factors for trisomy 9?
Trisomy 9 occurs randomly in most cases.
Research has found that if either one of the parents has a condition called balanced translocation, which affects chromosome 9, there is an increased risk of having a baby with partial trisomy 9. However, the occurrence of partial trisomy 9 is relatively rare compared to the other types of trisomy 9.
What are the symptoms of trisomy 9?
Signs and symptoms of trisomy 9 vary depending on the type. Some signs can be detected before a baby is born, whereas others may not be apparent until after the baby is born.
Symptoms of trisomy 9 that are noticeable after a baby is born include:
- Breathing and feeding difficulties at birth
- Characteristic clinical features, such as:
- Cognitive disabilities
- Congenital heart defects
- Developmental delay and growth problems
- Displaced joints
- Failure to grow well
- Heart murmurs
- Kidney cysts
- Limited hip abduction
- Overlapping fingers
- Problems with vision
- Rocker bottom feet
- Severe intellectual disability
- Short sternum and stature
- Underdeveloped genitalia
- Undescended testes in male infants
- Webbed neck
How is trisomy 9 diagnosed?
Trisomy 9 is often diagnosed after a miscarriage when routine karyotyping is performed:
- Chorionic villus sampling or amniocentesis: Karyotype can be done to confirm the diagnosis of trisomy 9 during pregnancy using placental tissue from chorionic villus sampling or through amniocentesis.
- Prenatal ultrasound: Most common findings during prenatal ultrasound include fetal heart defects and malformations of the brain and spinal cord.
Science Direct. Trisomy 9. https://www.sciencedirect.com/topics/neuroscience/trisomy-9
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