Hidden Genes: What people planning for pregnancy should know about carrier testing
Even people with no family medical history of genetic diseases may want to consider one type of genetic testing called carrier testing.
What is carrier testing?
Carrier testing is a type of genetic testing for people who are pregnant or planning for pregnancy. The greatest chances for most pregnancies is a healthy baby. However, some inherited diseases result from genes that are hidden in the family, so even people with no family history of serious health problems, birth defects, or genetic diseases can still have a chance to have a baby with an inherited disease. Carrier testing looks for some of those hidden genes.
Genes are made up of DNA, the blueprint that determines how a body grows, develops, and functions. Genes are passed from one generation to the next in a family. Sometimes there is a difference in a gene that causes the gene to not work properly. In some inherited conditions, if a person has one non-working copy of the gene and one working copy, she or he is called a carrier. Most carriers are healthy and would therefore never know that they are carriers.
Carrier testing looks at people’s DNA to find out if they are carriers for a particular set of inherited diseases. It requires either a saliva sample or a blood sample.
Who should consider carrier testing?
The greatest chances for most pregnancies is a healthy baby. Many prospective parents want to do everything they can to protect the health of their future child. Some people think that as long as they don’t have any family history of birth defects or inherited diseases, their children won’t either. Unfortunately, even healthy parents with healthy families can have children with genetic diseases. Therefore, anyone who is pregnant or planning for pregnancy could consider carrier testing.
The American College of Obstetricians and Gynecologists (ACOG) supports carrier testing before or during pregnancy.
Not all women's health, prenatal, or fertility care providers offer carrier testing, so some people will need to ask for carrier testing if they want it.
Is it best to wait until pregnancy to have carrier testing?
Although carrier testing is typically offered during the first trimester of pregnancy, that timing doesn’t give people the most options for family planning.
If carrier testing is done before pregnancy and a couple is found to have a higher chance for a known inherited disease, there are more options for family planning.
Many people don’t realize that in vitro fertilization (IVF) can be used to allow genetic testing on embryos, called preimplantation genetic testing or PGT, for certain inherited diseases. In this case, only the embryos that are unaffected by the inherited disease are transferred to the woman’s uterus. Other people may choose to adopt, use an egg or sperm donor, or even decide not to have children.
During pregnancy, there is an option of genetic testing on the fetus through amniocentesis or chorionic villus sampling. (These are called diagnostic tests, or prenatal diagnosis, compared to the screening tests described above.) Chorionic villus sampling (CVS) is done in the late first trimester. Amniocentesis is done in the second trimester. During CVS, a catheter is inserted through a woman’s cervix to obtain a small amount of tissue from the placenta. With amniocentesis, a thin needle is inserted through the mother’s abdominal wall and uterine wall to obtain a small amount of the fluid around the baby. (With CVS and amniocentesis, the needle does not touch the developing baby because the tissue or fluid contain cells, which have DNA.) A laboratory uses the tissue or fluid to look at the baby’s genes. If the fetus is found to be affected, the couple can choose to end the pregnancy or use the information to prepare for a child with potentially special needs. This preparation can include delivering at a medical center with a specialty focus, joining a support group, or learning about therapies.
What types of inherited disease are included in carrier testing?
There are two different types of inherited conditions that are included in carrier testing. The first type is autosomal recessive and the other is X-linked recessive.
Genes are located on chromosomes, and most people have 46 chromosomes, or 23 pairs. In each pair, one chromosome is from the mother and one is from the father. The 23rd pair are the sex chromosomes. Females typically have two X chromosomes, and males typically have one X and one Y chromosome.
In autosomal recessive conditions, the gene involved in the inherited disease is located on one of the first 22 pairs of chromosomes. If both parents are carriers for a condition, there is a 1 in 4 chance that a child could inherit two non-working copies of the genes and therefore have the condition.
The other type of inherited condition included on some carrier screening tests is X-linked recessive. In X-linked recessive conditions, the gene involved with the inherited disease is located on the X chromosome. If a male inherits the non-working copy of the gene, he will likely have the condition. However, a woman who has one working copy of the gene and one non-working copy of the gene is a carrier and is usually healthy. If a woman is a carrier of an X-linked recessive condition, she has a 1 in 4 chance to have an affected son. (Another way to say this is that if she has a son, he has a 1 in 2 chance to have the condition. If she has a daughter, her daughter will have a 1 in 2 chance of being a carrier)
There are a few conditions where female carriers can have symptoms or features of the disease. You might want to read our blog post about Fragile X testing as an example.
What can people planning for pregnancy expect from their carrier testing results?
Carrier testing can be “positive”, meaning that the person is found to be a carrier of at least one condition on the test. A carrier test can also be “negative”, meaning that the person was not found to be a carrier for one of the conditions on the test.
If a person is found to be a carrier of an autosomal recessive condition on a carrier test, the next step will be to test the person’s partner. If the partner is also a carrier for the same condition, then they have a 1in 4 chance to have a child with the condition in each pregnancy. If a woman is found to be a carrier for an x-linked recessive condition, then she has a 1 in 4 chance to have a child with the inherited condition in each pregnancy. People can consider options like using IVF for preimplantation genetic testing, using an egg or sperm donor, or prenatal diagnosis.
It’s important to realize with the larger carrier tests, about 25% of people will be a carrier for at least one condition and perhaps more than one. It’s important to consider before having carrier testing whether that information is helpful and whether one of the options might be valuable. Carrier testing is not required. If you question whether genetic testing is right for you, Mainstream Genomics’ Free Information Guide Saying No to Genetic Testing might be helpful.
If the person having carrier testing is not a carrier for one of the conditions on the test, then it is not typically necessary to test the person’s partner. However, it is important to realize that carrier testing cannot find all carriers. In fact, carrier testing finds most but not all carriers for the particular set of inherited diseases. (Most laboratories publish a list of the conditions included on its carrier testing.) There are also many other types of inherited diseases, even some that are autosomal recessive and X-linked recessive conditions, that are not included on any carrier testing. The Pregnancy Planning Family Screen is a good resource for assessing whether other genetic testing might be helpful.
Which carrier test is best for people who are pregnant or planning for pregnancy?
Labs now offer different forms of carrier testing. Some of these tests include over 250 different conditions and are called expanded carrier screening tests. Others are focused on a small number of diseases based on the person’s ethnic background or even on a specific disease that runs in a family. Recent studies have shown that using ethnicity to decide which carrier testing to offer may miss some carriers that expanded carrier screening would detect. ACOG currently recommends that all pregnant women be offered carrier screening for cystic fibrosis (an inherited lung disease), spinal muscular atrophy (a progressive nerve disorder), and hemoglobinopathies ( a blood disorder). However, ACOG also supports expanded carrier screening and encourages people to have carrier testing prior to pregnancy.
Some families know there is a specific inherited disease in their family and want to consider carrier testing. If that disease is not part of the standard offerings from labs, it is often possible to order carrier testing for that particular disease. The Pregnancy Planning Family Screen from Mainstream Genomics is a starting place for parents-to-be to assess the health of their families and decide whether carrier testing is right for them.
How do people who are pregnant or planning for pregnancy get carrier testing?
There are a few options for ordering carrier testing. A physician (or in some states, a nurse practitioner) can order carrier testing. However, many healthcare providers are not familiar with all the different types of carrier testing, or they may only offer carrier testing during pregnancy.
A genetic counselor, who is specially trained in genetics and is shown to be more familiar with genetic risks and genetic testing, can help coordinate carrier testing for people and their healthcare professionals.
Using the expertise of genetic counselors, Mainstream Genomics offers a service to help people who are planning for pregnancy determine which genetic services are best for them.
Some laboratories also allow people to order carrier testing online by connecting them to physicians online. Many of these online services include an optional conversation with a licensed and certified genetic counselor. To help determine which carrier testing to order, Mainstream Genomics’ Pregnancy Planning Family Screen collects family health history and provides a Personalized Screening Plan with any suggestions from a genetic counselor for genetic testing. Some carrier testing is available to people without a doctor’s order, but the FDA does not advise using them for medical purposes. These tests can be fun for people interested in genealogy but may not include the same conditions as physician-ordered tests. They also may miss some carriers that the physician-ordered tests would find.
Does insurance cover carrier testing?
Some insurance policies will cover carrier testing. Some policies will only cover carrier testing during pregnancy, but a woman or couple has fewer options at that time. Others will only cover carrier testing for a condition that is known in the family or recommended by medical associations for certain ethnic groups. Checking your own policy is important to avoid any surprises.
How much does carrier testing cost?
Some labs offer carrier testing for a cash rate as low as $250 for the woman and $100 for her partner. Some couples start with one partner and only pursue testing for the other partner if the first partner is found to be a carrier for an inherited condition. The cost billed to an insurance company can vary.
Ellizabeth Kearney, MS, CGC
Licensed Genetic Counselor
ACOG Committee Opinion, Carrier Screening for Genetic Conditions, March 2017.
ACOG and ACMG Commentary, Expanded Carrier Screening in Reproductive Medicine - Points to Consider. Obstetrics and Gynecology, 2015. 125(3): 653-62.
Lazarin and Hague. Expanded carrier screening: a review of early implementation and literature. Seminars in Perinatology, 2016.
Kasenitt et al. Genetic ancestry analysis on >93,000 individuals undergoing expanded carrier screening reveals limitations of ethnicity-based medical guidelines. Genetics in Medicine, 2020.