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What to Know About Carrier Screening

At Pacific Fertility Center, we are committed to providing our patients all the information they need to make informed choices and to have the very best chance of delivering one healthy baby at a time.

before you give birth, and even before you transfer an embryo.

Carrier screening is an advanced—yet simple—blood test that allows screening for many genetic diseases at once, rather than screening for each individually. This type of screening not only is more efficient and less costly, it’s also more accurate—all reasons we recommend it to our patients.

What Carrier Screening Tests

Carrier screening tests for two types of genetic conditions, recessive and X-linked. To understand these conditions, it first helps to understand a little more about genetics.

Within your cells, you have 23 pairs of rod-shaped structures called chromosomes—half came from your mother’s egg and half from your father’s sperm. Each chromosome contains hundreds of genes, which are segments of DNA with the instructions your cells need to grow and develop. A mutation occurs when a change in the gene’s DNA causes the gene to malfunction, which can then cause disease.

Recessive conditions. Both men and women can be carriers of recessive conditions. Carrier screening can identify whether either or both prospective parents are carriers.

Both men and women have two copies of recessive genes, which are located on chromosomes 1 through 22. If you have a mutation in one copy of these recessive genes, but not the other, you are a carrier of a condition. If your partner is not a carrier, your child will not be affected because one copy of the gene works normally. However, if a child inherits the mutation in both copies of the recessive gene—one from the mother and one from the father—the child will have the condition. One example of a recessive genetic disease is Cystic Fibrosis which can lead to serious lung problems for affected children.

X-linked conditions. The 23^rd pair of chromosomes is the site of the sex chromosomes—X and Y. Two X chromosomes create a female baby, while a combination of an X and Y chromosome creates a male.

X-linked genes are on the X chromosome. A woman with a mutation in only one of her X chromosome genes is a carrier of an X-linked condition. It is extremely uncommon for a woman to have two mutated X chromosomes as one would have to come from both her father and her mother and her father would have been affected. Children, typically males, of female carriers of X-linked conditions are at risk for having the disorder, because males have a 50% chance of inheriting the mutated X chromosome from his carrier mother. The same is true of girls but they also inherit a normal X chromosome from their father so the worst case scenario for daughters is to be a carrier like her mother. Because they only have one X chromosome, males are typically not carriers, they can either be affected with the disorder or unaffected. They either are healthy or have the condition. That’s why PFC doesn’t usually test healthy men for X-linked disorders. One example of an X-linked disorder is Factor VIII deficiency, or hemophilia. Another examples is Duchenne Muscular Dystrophy.

The Testing Process

To do carrier screening, PFC uses Sema4 Expanded Carrier Screening Panel for its high accuracy, relatively low cost, and extensive list of diseases screened. If you have questions about your screening options or want to discuss your results once they become available, Sema4 offers free phone consultations with genetic counselors. Sema4 is also able to bill your insurance directly for testing. Talk directly with Sema4 to learn whether your insurance covers the full cost of carrier screening.

For the test, we obtain a small DNA sample from you, usually from a blood sample, and send it to Sema4 for processing. You can expect tests results within about two weeks. PFC strongly recommends that both reproductive partners be tested at the same time so we can review your test results simultaneously.

A negative test result. If your results are negative (normal), none of the genes tested on the panel carry mutations in their DNA. No test is capable of detecting mutations 100 percent of the time, though, so a negative result doesn’t completely rule out the chance you are a carrier. However, it significantly reduces the risk.

A positive test result. Before undergoing carrier screening, it’s important to understand that about 66 percent of people will receive a positive test result. That’s because we are all carriers for at least one genetic condition. This high rate of carriers on the screening is why we recommend both partners be screened at the same time.

If you’re healthy, it may seem confusing or alarming to receive a positive test result. But with few exceptions, being a carrier of a recessive or X-linked genetic condition is unlikely to affect your own health. However, being a carrier of a few conditions could possibly mean you are at higher risk of adult-onset conditions such as a neurologic disorder. If this is true for you, our genetic counselor will discuss this with you.

Here’s what positive results may mean for your child:

• If one partner is a carrier, but the other is not, your child’s risk of disease is very low, but not zero. Talk this over with your genetic counselor to see if any extra testing is warranted for the partner who tested negative.
• If both you and your partner are carriers for the same recessive genetic condition, your child has a 1 in 4 (25 percent) chance of being affected. It may help to know that the risk of both prospective parents being carriers of the same genetic disease is less than 1 percent.
• If you are female and a carrier of an X-linked condition, your male offspring have a 1 in 2 chance of being affected and your female offspring (1 of 2 will be carriers) may be at risk for a milder form of the condition.

Next steps. If the test results show your child is at risk for disease, we can address this directly by offering preimplantation genetic diagnosis (PGD) of your embryos as part of the IVF process. This way, you can choose not to transfer embryos with the disease.

No carrier screen can test for all genetic diseases. Therefore, if a family member has a genetic condition, be sure to discuss this with your doctor or genetic counselor. We will want to make sure we offer you additional tests, if warranted.

Sources

1. Sema4 website.
2. ACOG: “Preconception Carrier Screening.”

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