Prenatal Genetic Carrier Screening

What is Carrier Screening?

You may have heard of carrier screening from your OB/GYN or midwife. You may also have heard about it from a friend or family member. Carrier screening is a test typically ordered by your midwife or OB/GYN. The test requires a blood draw and analyzes your DNA. The test is looking for markers in your DNA that signal that you are a carrier for a genetic disorder. Some examples of these genetic disorders are listed below.

  • Cystic Fibrosis
  • Tay Sachs Disease
  • Fragile X Syndrome
  • Spinal Muscular Atrophy
  • Gaucher Disease

A Quick Lesson in Genetics

To understand carrier screening I am going to give you a brief tutorial in genetics. Your DNA comes in 26 pairs of chromosomes- making up 46 individual chromosomes. One chromosome in the pair comes from your mom and one chromosome comes from your dad. Each chromosome in the pair codes for the same basic stuff. For example, if chromosome pair #18 has the code for eye color, one chromosome could say brown and one could say blue, but they both code for eye color. The chromosome that codes for brown would mask or hide the chromosome that codes for blue and your eyes would come out brown.

Genetic Mutations

Ok, so back to mutations. In your DNA you could have a mutation on one chromosome that codes for cystic fibrosis, but your other chromosome in the pair could be normal. The normal chromosome would mask, or hide the abnormal chromosome and you would be healthy. This is what we call a “silent carrier. You still carry the abnormal mutation, but it is not being expressed.

Now, when you have a baby your body randomly selects which chromosome of the pair you pass onto your baby. The same occurs for your partner to make a new 23 pairs. Now if you pass on the chromosome with the mutation and your partner passes on a chromosome without a mutation, your baby is also a silent carrier. However, if your partner is also a silent carrier and also passes on a chromosome with the mutation your baby will have the disease. In this case that is cystic fibrosis. You have a 25% chance of having a healthy baby who isn’t a carrier, a 50% chance of having a baby who is a silent carrier and a 25% chance of having a baby with the disease. You can see this depicted below.

How Does Carrier Screening Work?

Let’s go back to carrier screening. So it is possible to be healthy and not know you are a silent carrier of a serious disease. This is where carrier screening comes in. Carrier screening analyzes your blood to see if you are a carrier of a disease.

Most people are a carrier for something. This is not a reason to worry or panic. Even if your test comes back and you are a carrier for something awful, it is only a problem if your partner is also a carrier.

If you are a carrier the genetic counselor (or midwife or doctor) will reach out to you to have your partner tested. Now depending on what you were a carrier for, you may be able to send in a saliva sample or you may need to come in for a blood draw. This test is exactly the same. This test is looking to see if your partner is a carrier of any genetic mutation that leads to an identifiable disease.

What Do Your Results Mean?

  • Your results may come back and say you are not a carrier for any genetic disease. Congratulations, you are done.
  • More likely, your results may come back and say you are a carrier for a genetic disease and you need to have your partner tested. So you partner completes the same test you did. You may find out your partner is not a carrier for any genetic disease. Congratulations, you are also done.
  • However you may find out your partner is a carrier for a genetic disease. If this is the case you will most likely set-up a consultation with a genetic counselor. This can be in person of over the phone.

If the disease or diseases your husband is a carrier for are not the same as the disease or diseases you are a carrier for you are most likely in the clear.

However, if you are both carriers of the same disease it does not mean you need to split up and find a new partner. You still have options.

What Happens if my partner and I are Carriers of the Same Genetic Disorder?

Preimplantation Genetic Diagnosis (PGD)

This depends on whether or not you are already pregnant. If you are not yet pregnant you could opt for preimplantation genetic diagnosis (PGD). PGD is expensive and does require you to undergo IVF. If you and your partner screened positive on carrier screening your doctor will first want to create a “probe” for your specific mutations. The lab is then able to use this probe to search the embryo’s DNA for your specific mutations before implantation.

With PGD, just like with IVF, you first need to have your eggs retrieved and then fertilized with sperm to form embryos. The embryos are then allowed to grow and multiply under optimal conditions for a few days until there are enough cells for testing.

Once the embryos are large enough for testing, the lab removes DNA from the embryos and tests the DNA for a general panel of genetic mutations as well as uses your probe to look for your mutation. The lab then selects only the embryos that are genetically normally and lacking your mutations for implantation.

Diagnostic Testing

If you are already pregnant you can undergo diagnostic testing to find out if your baby carries two copies of the mutation. If you and your partner screened positive on carrier screening your doctor will want to create a probe for your specific mutations. There are two diagnostic tests available.

The first is called Chorionic Villus Sampling (CVS). CVS can be done between 10 and 14 weeks. With CVS the doctor removes a sample of DNA from the placenta and tests it for mutations. The lab can use your probe to detect the presence of your specific mutations. This will tell you if your baby has the disease, is a silent carrier, or has 2 normal copies of the gene.

The second is called Amniocentesis. Amniocentesis is similar to CVS in that it tests for the same things. However, doctors typically perform amniocentesis between 15 and 20 weeks. With amniocentesis your doctor is taking a sample of the amniotic fluid and the lab tests this sample for mutations. The lab can also use your probe to detect your specific mutations.

Who Pays for Carrier Screening?

Many insurance companies will pay for the majority of carrier screening. Some insurance companies may consider carrier screening medically unnecessary or require pre-authorization. If your insurance requires pre-authorization your doctor’s office should be aware. In either case, you can always be on the safe side and contact your insurance company to find out what your cost will be.

The cost of carrier screening may also go towards your deductible. However, if you are pregnant or planning on becoming pregnant this year, you will likely hit your deductible anyway.

Carrier screening is a test every couple should consider. It can detect mutations that CVS and amniocentesis alone cannot detect. Carrier screening could be your only indicator that your child could have a serious genetic disorder. Contact your doctor or midwife today to get tested.

If you are thinking of becoming pregnant, make sure you check out my article on “What are the Best Prenatal Vitamins?”


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