Prenatal Testing for Genetic Disorders

If you’re thinking about getting pregnant and wonder if you can be tested for some of the silent , recessive genetic disorders, then keep reading.

January was National Birth Defects Prevention Month, which is a great opportunity to discuss carrier testing for recessive disorders. Carrier screening is a type of genetic test that can tell you whether you carry a gene for certain genetic abnormalities that cause disease. The testing finds out if a prospective parent, who has no illness, is carrying 1 copy of a defective gene. It’s only when both parents contribute 1 copy of the defective gene that conditions like sickle cell anemia, cystic fibrosis, and spinal muscular atrophy are more likely. 

What Are the Odds?

Even when both parents are carriers, because of Mendelian genetics, there is only a 1 in 4 chance of them both passing on the defective gene and having an affected baby.

Each parent has 2 genes and if they are carriers, 1 is defective and 1 is normal.

In this case 

B = a defective gene 

B =  normal gene

Mom – Bb: carrier               Dad – Bb: carrier

Kids: 

1 – bb: no disease, not a carrier

2 – Bb: no disease, is a carrier

3 – Bb: no disease, is a carrier

4 – BB: Disease

How is Carrier Screening Done?

A sample of blood, saliva, or cells from inside the mouth are analyzed for changes in chromosomes, DNA, or proteins, depending on the disorder. The partner who is at higher risk for testing positive for the defective gene is usually tested first; if the result is negative no further testing is needed on the other partner. 

Carrier screening can be done before deciding to become pregnant, but can also be done during pregnancy. Getting testing results before pregnancy gives you and your partner more time and more options.  

Carrier screening is available for:

• Cystic fibrosis
• Fragile X syndrome
• Sickle cell disease
• Tay–Sachs disease
• Hemophilia
• Spinal Muscular Atrophy (SMA).

Targeted and Expanded Screening Guidelines

Targeted screening means that you are tested because of your ethnicity or family history. Certain ethnic groups and races have a higher rate of carriers of specific genetic disorders than others, so carrier screening for these disorders may be recommended. This also is called ethnic-based carrier screening. People with family members who have a specific disorder would be candidates for screening no matter their race or ethnicity. 

There are companies that offer what they call expanded carrier screening, where from a single sample they test from their own screening panel. These panels may test for over 200 different abnormalities without regard for ethnicity or race. Usually, the focus of these panels are severe disorders that begin in childhood. 

Though there is some controversy over the use of expanded testing, many families and many healthcare providers recommend expanded screening for everyone so that they don’t miss conditions that could have been detected and don’t have to try to determine someone’s ethnicity. 

Basic Carrier Screening recommended by American College of Obstetrics and Gynecology (ACOG): 

All ethnicities

All ethnicities can consider carrier screening for cystic fibrosis,

spinal muscular atrophy, and Fragile X syndrome.

African ancestry

Carrier screening is recommended for thalassemia, thal-uh-SEE-me-uh, an inherited blood disorder that causes your body to have less hemoglobin than normal causing anemia, and sickle cell disease.

Southeast Asian or Mediterranean ancestry

Carrier screening is recommended for thalassemia.

French Canadian/Cajun/Creole ancestry

Carrier screening is recommended for Tay-Sachs, a disease caused by the absence of an enzyme which then affects the function of the nerve cells leading to blindness, paralysis, and death.

Jewish ancestry

Carrier screening is recommended for cystic fibrosis, Tay-Sachs disease, Canavan disease, a gene-linked neurological disorder in which the brain degenerates, and familial dysautonomia, a rare, progressive, recessive genetic disorder of the autonomic nervous system.

Can these Tests be Wrong?

In a small number of cases, a test can result in a false-negative–you actually do have the recessive gene in question–or a false-positive result when you do not have a gene for a disorder. 

It is recommended that you see a qualified genetic counselor to discuss what tests will be taken; many counselors suggest a visit before conception. A genetic counselor will help you understand why you should get genetic testing–or not. Your counselor will also be able to offer guidance should you get a positive result. 

Difficult Choices

If a couple is identified as having increased risk of passing along a serious genetic defect, they can consider many options. Some decide not to have children, or take a risk on another pregnancy. Another option is to use preimplantation genetic diagnosis (PGD), this is when harvested embryos are tested before being implanted in the mother’s uterus. Or, a donor egg or donor sperm could be used. Couples with high risk of having a severely affected child may opt to terminate the pregnancy.