Tests Performed Before or During Pregnancy
Most people have 46 chromosomes in the cells of their bodies, 23 inherited from the mother and 23 from the father. The chromosomes carry all of a person’s “genes” and the genes determine many of our physical and health characteristics. Chromosome testing is offered prenatally by amniocentesis or CVS to check the baby for Down syndrome (caused by an extra chromosome, see below) or other chromosome disorders. It may be offered to couples who have a history of miscarriage to check for more subtle chromosome changes which can sometimes be responsible for repeated pregnancy loss.
Cystic fibrosis (CF) carrier testing
Cystic fibrosis (CF) is a life-long disease that causes problems with digestion and breathing. A person can only have Cystic fibrosis if they inherit a mistake in the copy of the CF gene inherited from both their mother and father. CF carriers have a mistake in one copy of the gene and do not have any CF symptoms. Even if no one in your family has ever had CF, you could still be a carrier. The chance to be a carrier is based on your ethnic background. If you are white or of European or Ashkenazi Jewish descent, your chance of being a carrier is 1 in 29 or about 3%. If you are Hispanic your risk is 1 in 46. African Americans have a risk of 1 in 62 and Asian Americans 1 in 90. Your baby can have CF only if both parents are carriers. If you have a carrier test for CF and an abnormal gene is found, your partner can be tested. If your partner also has an abnormal gene, there is a 1 in 4 chance (25%) that your baby will have CF. You can have a test during the pregnancy (amniocentesis or chorionic villus sampling) to see if your baby will have CF or not.
Sickle cell disease and Thalassemia are both abnormalities of hemoglobin. Hemoglobin is the substance in blood that carries oxygen and gives blood its red color. Sickle cell anemia is usually diagnosed early in childhood due to low blood count and frequent attacks of pain called crises. It is treated with daily penicillin, folate, pain medicine as needed, and occasionally transfusions. Thalassemia is also usually diagnosed early in childhood due to a very low blood count. It is a very serious disease and is treated with blood transfusions as needed. Both sickle cell disease and thalassemia are caused by a mistake in both copies of the hemoglobin A gene. Carriers have a mistake in one copy of the gene and are generally healthy. Sickle cell is found primarily in African Americans and Africans, and sometimes in people from around the Mediterranean Sea and Caribbean Islands, while thalassemia is more common in people whose ancestors come from around the Mediterranean (Greece, Italy, Turkey, and the Arabian Peninsula), Asia, Africa, and the West Indies. Your baby can have these hemoglobin diseases only if both you and your partner are carriers, and even then, the chance to be affected is only 25%. Prenatal testing by amniocentesis or CVS is available.
Tay-Sachs Disease and Other Ashkenazi Disorders
Certain genetic disorders are more common in people of Ashkenazi (Eastern European) Jewish descent. These include cystic fibrosis (discussed earlier), Tay-Sachs disease, Canavan disease and several others listed below. Each of these diseases is caused by different genes, but all are inherited the same way. In order to inherit one of these disorders, there must be a mistake in both copies of the gene that is responsible for the disorder. Carriers have a mistake in one copy of the gene and do not have any symptoms. Your baby can have one of these diseases only if both parents are carriers for that disease.
- Canavan disease is a progressive disorder causing paralysis and blindness. There is currently no treatment and children usually die in early childhood.
- Niemann-Pick disease causes poor growth and progressive mental and physical deterioration. There is currently no treatment and children usually die by 4 years of age.
- Fanconi anemia is diagnosed in early childhood with low blood count, short height and learning disabilities or mental retardation. People with Fanconi anemia are also at high risk for cancer, especially leukemia.
- Bloom syndrome causes poor growth, immune system problems and a high rate of cancer. Patients usually die by the age of 30 due to cancer.
- Familial dysautonomia is a nervous system disorder causing vomiting, sweating, decreased pain sensitivity and unstable blood pressure or temperature. About half of all people who have it die before age 30.
- Glycogen storage disease type 1a causes severe low blood sugar, bleeding, enlarged liver and delayed growth. It is treated by a strict special diet and continuous tube feedings of glucose.
- Maple syrup urine disease (MSUD) causes certain substances to accumulate in the blood. If untreated, mental retardation, seizures and death would occur. It is treated by a strict lifelong special diet.
- Gaucher disease is caused by a deficiency of the enzyme glucocerebrosidase and causes enlargement of the liver and spleen and destruction of the bones. The more severe form can affect brain development as well. There are also mild forms of Gaucher disease that cause very mild symptoms.
If you are of Ashkenazi Jewish descent, your care provider may offer testing for these disorders. If you test positive for one of them, your partner can be tested for that disease. If your partner also is a carrier, there is a 1 in 4 chance (25%) that your baby will have the disorder you both carry and prenatal testing is available.
If you are French Canadian or Cajun, your care provider may offer you Tay-Sachs testing.
Fragile X is an inherited condition that causes mental retardation. It is more common and more severe in males, but can occur in females too. Symptoms range from mild learning disabilities to severe mental retardation and autism. Fragile X is caused by a repeating sequence of the genetic code in a gene on the X chromosome. Carriers have 50 to 200 copies of this sequence and do not have any symptoms, but the number of copies can expand when it is passed from a carrier parent to a child. Affected individuals have more than 200 copies of the sequence. All males and approximately half of the females with over 200 copies are mentally retarded. If you have a family history of mental retardation, your care provider may offer you Fragile X carrier testing. If you are a carrier, prenatal testing is available.