Genetic Carrier Screening & Pregnancy
To the Point: Prenatal carrier screening involves a blood draw on mom and/or dad to determine if they carry genetic changes that could be passed on to the baby. Most carrier screening is done for recessive conditions. With recessive conditions a carrier does not have any symptoms, but if they are both carriers of the same genetic condition there is an increased chance of having a child with that genetic condition. Historically, the conditions screened for have been offered based on an individual or couple’s ethnic background, or family history, which is still often the case. However, many providers now offer what is called expanded carrier screening, which may include testing for dozens to hundreds of conditions.. Carrier screening may be performed prior to or during the pregnancy.
So, why do they call it carrier screening in the first place? All of us are what the genetics world would call, “carriers” of certain genetic conditions (in fact, we all are probably carriers of 5-10 recessive genetic conditions or more and just never know about it). As you may recall or can go back and review in genetics 101, we have 2 copies of each gene, one from our mom and one from our dad. When we are a “carrier” of a genetic condition, this simply means that one of the copies out of the 2 genes has a change or mutation and is not working as usual. But, the other copy of the gene that is working compensates or makes up for the one that isn’t working. So we don’t have the genetic disorder or any symptoms; we are a “carrier” of the disease. Again, there are typically no health problems associated with being a carrier of a mutation. However, as a carrier, there is a chance that your children could inherit that genetic disorder.
Most carrier screening looks for genetic disorders that are inherited in an autosomal recessive manner. Simply put, this means that in order for a child to get the condition, both mom AND dad must be “carriers” of it. If you are a carrier but your partner is not, we are not worried about your child getting the condition (in general). However, if you AND your partner are carriers of the condition, there is a 25% chance or ¼ chance for each of your children to inherit the condition.
Other carrier screening looks for genetic disorders that are inherited in an X-linked manner. This means that the gene involved is on the X chromosome. Since women have 2 X chromosomes and men have only 1 X chromosome, this type of condition affects men and women differently. Women who carry the condition may have very mild or no symptoms, whereas, men may have the full-blown condition. So, if a woman is a carrier of an X-linked condition, she will have a 50% chance of passing it on to her children, but boys will have the symptoms associated with the genetic condition and girls may not (or symptoms may be very mild).
Should I have genetic carrier screening or not?
Genetic carrier screening is your choice; it’s optional. As with other prenatal testing options, there are some questions to consider as you contemplate this decision. In order to answer some of these questions you may need to do a bit more reading on the conditions…that information is below as well.
Screening for Common Disorders
Offered to EVERYONE, regardless of ethnic background.
Some genetic disorders are relatively common and carrier screening may be offered to you regardless of your ethnic background. Examples of common disorders for which carrier screening is routinely offered to everyone include:
Screening for another condition, Fragile X may be offered, however, the American College of Medical Genetics and other professional societies do not necessarily recommend offering this to all patients unless there is a family history or some other indication for doing so. This being said, carrier screening is your choice and you can choose to have this testing even if you don’t have an indication that makes it higher risk in your family.
Screening for Ashkenazi Jewish Disorders
Offered to couples where at least 1 of the partners has some Ashkenazi Jewish ancestry.
There are a number of genetic disorders that are more common in people of Ashkenazi Jewish descent/ancestry; however, they may occur in people of other ethnic backgrounds as well. Many of these disorders can have serious affects on the patient’s quality of life, some of which may result in shortened life expectancy. The following disorders are recommended by the American College of Medical Genetics & Genomics to be offered to couples who are planning a pregnancy or currently pregnant where at least one partner has Ashkenazi Jewish descent:
There are other conditions that are increased in risk in the Ashkenazi Jewish population for which you may be offered screening.
*Tay-Sachs disease is also increased in risk among individuals of French-Canadian, Cajun & Irish ancestry and should be offered to individuals with this ethnic background.
Screening for Hemoglobinopathies
The American Congress of Obstetricians and Gynecologists recommends that screening for hemoglobinopathies be offered to couples where at least 1 of the partners has some ethnic background including the following:
Hemoglobin diseases, called hemoglobinopathies, are a group of disorders characterized by abnormal or decreased production of hemoglobin, the molecule that carries oxygen throughout the body. The severity of these diseases ranges from mild to severe, depending on the type of hemoglobin defect. The most common hemoglobinopathies are the following:
Expanded Carrier Screening
Offered by some providers but if not it may be available upon your request. Expanded carrier screening is available to all individuals, regardless of ethnicity. Your insurance may or may not cover this testing.
Expanded carrier screening looks for hundreds or thousands of changes or mutations in your genes or DNA that are involved in over 100 genetic conditions with the goal of identifying “carriers”. A “carrier” is someone who has a change or mutation in a specific gene but usually doesn’t have any symptoms. Some conditions, termed autosomal recessive, require both parents to be a carrier in order to have a child with the condition. In these cases, the chance of having a child with the condition when both parents are carriers is 25% with each pregnancy. Other conditions are X-linked and mom may carry the genetic condition without symptoms but has a 50% chance of passing it on to her children, with boys being more affected than girls (girls may have no symptoms or very mild symptoms).
Some of these conditions are more severe, some mild, some have treatment available and some don’t. There is a very small chance that you could be found to have a genetic condition through this test as well (depending on what laboratory your doctor uses and what conditions are screened for). In general, this test screens for all of the conditions commonly offered based on ethnic background (CF, spinal muscular atrophy, common Ashkenazi Jewish diseases, hemoglobinopathies, etc.). It also often includes Fragile X testing, which some of the medical professional societies DO NOT recommend as a population-based screen (versus being offered if there is something in the family history that is concerning).
The labs offering expanded carrier screening may or may not be as comprehensive in their analysis (look for as many changes/mutations in each gene) as labs that do not focus on expanded carrier screening. What this means is that if their testing is not as comprehensive and you get a normal/negative carrier test result, the chance that you could still be a carrier (this is called residual risk) may be higher.
As you can imagine, there are many viewpoints on the use of this test and many ethical issues involved. This test begs the question, “is bigger better?” Some would argue yes, some would argue no. There are pros and cons to this testing and it is worth discussing these more with your provider if you are thinking of pursuing expanded carrier screening.