What is prenatal genetic testing?
Genetic testing in pregnancy shows the likelihood that an unborn child will have certain inherited health conditions. Genetic testing can also determine, with as much certainty as possible, whether an unborn child has certain genetic disorders or birth defects. These prenatal tests can inform expectant parents about the health of the fetus so they can make informed choices.
The tests can also help your health care provider tailor their care for you and increase your chances for a healthy pregnancy.
Prenatal screening isn’t mandatory — it’s optional. And it’s very important to make an informed decision about whether or not to undergo screening.
Pros and cons of prenatal genetic screening
Not all genetic testing has to be done during pregnancy. If two partners both belong to an ethnic group with a higher risk of certain genetic disorders, then the couple may undergo a test to see if they carry certain genes. This is called a carrier screening. Many genetic conditions such as Tay-Sachs, sickle cell disease, and cystic fibrosis require that both parents be carriers of the gene or genes.
This information can affect family planning choices, including whether to have children or to undergo IVF with preimplantation genetic screening.
Couples who discover that their baby has a genetic defect are generally referred to a genetic counselor to help them understand the condition and their options.
Genetic screening in pregnancy can determine the risk for birth defects and genetic conditions, but an actual diagnostic test can tell for sure. Couples who discover that their baby has a genetic defect are generally referred to a genetic counselor to help them understand the condition and their options.
Types of prenatal genetic testing
There are a couple of different types of genetic tests that are conducted during pregnancy.
Screening tests will determine whether the fetus has a high risk for a certain condition. Here are a few types of screenings:
- First trimester combined screening is usually performed between weeks 10 and 13 of pregnancy. First trimester combined screenings include ultrasound (nuchal translucency ultrasound measurements) and maternal blood test for PAPP-A and beta-hCG. The results of the blood test, ultrasound, maternal age, and gestational age of the fetus are then used to calculate the risk of genetic disorders like Down Syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13).
- Second trimester screening is performed between weeks 15 and 25 of pregnancy and includes a maternal blood test (a quad test or triple test depending on which state you live in) and ultrasound. The International Society of Ultrasound in Obstetrics and Gynecology recommends performing an ultrasound between 18–22 weeks of pregnancy as part of routine prenatal care. Ultrasound at this point in the pregnancy can detect neural tube defects, anencephaly, congenital heart defects, renal agenesis, gastrointestinal defects, and other conditions.
- Modified Sequential Screening is a combination of the first trimester and second trimester screening. The results are first reported after the first trimester sample has been submitted, and a final determination is made after the second trimester sample. Sequential screening has a higher detection rate than individual screen results.
- Cell-free DNA screening may be performed from 10 weeks on through pregnancy. The test measures small fragments of fetal DNA in the mother's blood and can determine the chance of a chromosomal problem. This test is not recommended for those pregnant with multiples.
Diagnostic tests check for the presence of certain genetic conditions, unlike a risk-assessment screen. They’re invasive tests and can diagnose certain fetal conditions with a high degree of accuracy.
Diagnostic tests check for the presence of certain genetic conditions, unlike a risk-assessment screen.
These are some common diagnostic tests:
- Amniocentesis is usually performed between 15 and 20 weeks of pregnancy. A thin needle is inserted into the womb, and a small amount of the amniotic fluid that surrounds the fetus is removed under ultrasound surveillance. The fluid is then tested for genetic birth defects and abnormal chromosomes. This can also be used as a paternity test while pregnant. Leakage of amniotic fluid and slight bleeding can occur after amniocentesis, but in most cases it stops on its own. There is a very small chance of pregnancy loss because of amniocentesis
- Chorionic Villus Sampling (CVS) testing is conducted using a sample of the placental tissue. This test can be performed between 10 and 13 weeks of pregnancy, which is one advantage it has over amniocentesis. However, there is a higher risk of pregnancy loss with CVS.
You may choose to have fetal genetic diagnostic testing done after your initial screening. Many parents choose to do so if the initial screening detects a higher risk of any abnormalities. Diagnostic testing is more comprehensive than screening and can detect, with as much certainty as possible, conditions caused by an extra chromosome or health conditions that result from missing or damaged chromosomes.
Many parents choose to have fetal genetic diagnostic testing done if the initial screening detects a higher risk of any abnormalities.
Many inherited conditions can be identified through genetic diagnostic testing during pregnancy; however, this testing does pose a risk for the fetus, including miscarriage. While this risk may be low, you should have a personalized consultation with a genetic counselor or health care professional to analyze your family history and the results of your prenatal screening tests. At this consultation, you’ll be able to discuss the information from the tests and your options.
Who should undergo genetic testing during pregnancy?
According to the American College of Obstetricians and Gynecologists (ACOG), all pregnant women, regardless of age or risk level, are recommended to undergo first-trimester screening. Aside from this, genetic testing is typically offered to couples who have a family history of certain conditions or are identified as having a higher risk for a baby with a birth defect or genetic health condition.
All pregnant women, regardless of age or risk level, are recommended to undergo first-trimester screening.
ACOG recommends counseling patients on the prenatal testing options available, with explanations of their relative advantages and disadvantages, detection rate, positive and false positive rates, and limitations. Patients should also be counseled about having diagnostic testing. After counseling, patients may decline for any reason.
While most women have access to these tests, whether to undergo them is a personal decision. If possible, you may want to discuss it with your partner. Your decision may include several factors, including your potential for a baby with a genetic abnormality or a family history of birth defects. Personal experience with a family member who has a birth defect or genetic abnormality can also play a role in the decision to have genetic testing done.
It’s important to note that some birth defects occur through spontaneous mutation of DNA. That means you and your partner may have no family history of genetic disorders and still have a baby with an extra chromosome or with damaged chromosomes that result in a health condition.
While most women have access to these tests, whether to undergo them is a personal decision. If possible, you may want to discuss it with your partner.
Other considerations are your financial resources or your network of family or friends to support you if you have a baby with a genetic disorder. Some of these conditions require around-the-clock caregiver services, and, depending on your resources or ability, you may not feel that you’re in a position to provide the care needed. Other people may wish to know in advance what care their baby will need to begin establishing these resources or downsizing their lifestyle to make more funds available for ongoing medical care. Finally, some women wish to have a paternity test before birth.
You may wish to have as much information as possible before the baby is born, and knowing for certain whether your baby is affected by genetic abnormalities can help you get in touch with support groups or begin finding physicians who specialize in the care of your child’s illness if necessary. Other women may opt not to have genetic testing done, preferring to focus on anticipating positive health news.
When is the right time to undergo genetic testing?
Women have several options for genetic testing. You don’t necessarily have to be pregnant to decide to have genetic screening. If you and your partner share a similar family history of conditions such as Down syndrome, Tay-Sachs, or cystic fibrosis, for example, then you may both wish to undergo carrier screening before deciding whether to become biological parents.
As we said earlier, most women undergo first-trimester screening. This is optional, but your obstetrician may recommend it. It’s generally done after week 10 of pregnancy and strongly recommended if your pregnancy is considered high-risk.
If you didn’t undergo first-trimester screening, you may opt for a maternal serum quad screening during your second trimester. This involves a blood draw and an analysis of certain protein markers in your blood that indicate if the fetus has trisomy 13 (Patau syndrome), trisomy 18 (Edwards syndrome), or trisomy 21 (Down syndrome). Neural tube defects, such as spina bifida (where the neural tube surrounding the spine hasn’t closed) are also part of the test. The quad screen may be completed any time between weeks 16 and 23 of pregnancy with the optimal time frame at weeks 16–18 of gestation.
If you and your partner share a similar family history of conditions such as Down syndrome, Tay-Sachs, or cystic fibrosis, for example, then you may both wish to undergo carrier screening before deciding whether to become biological parents.
You may also choose to have a diagnostic screening done between weeks 18–22 of gestation. This is a comprehensive scan that checks every major system in your baby’s body. Often, certain conditions become apparent on the visual screen, and if your doctor notices the presence of certain physical conditions, they may offer you the opportunity for diagnostic testing to correctly identify your child’s health condition.
What to expect from the prenatal genetic testing procedure
Each type of prenatal genetic testing has a different process. For genetic screening, you may simply submit a family health history and undergo a blood test. For certain types of genetic testing, both partners may have a blood sample taken to determine if they’re carriers for certain conditions, such as sickle-cell anemia or cystic fibrosis. You may also opt for a paternity test before birth, which is more invasive for the fetus.
Each type of prenatal genetic testing has a different process. For genetic screening, for instance, you may simply submit a family health history and undergo a blood test.
During the screening process, your blood will be analyzed for certain genetic markers. Once the testing is complete, you’ll have a counseling session to discuss the results with your doctor. At this point, if the screening has indicated high risks of genetic abnormalities, then you’ll have options for diagnostic testing.
Choosing to have carrier screening, prenatal screening testing, or prenatal diagnostic testing is a personal choice. Each woman and each pregnancy is different, and while you may have a family history of certain illnesses, it’s also true that some genetic abnormalities occur as a random mutation. If you’re considering prenatal genetic screening, schedule an appointment with your OB-GYN or a genetic counselor who can guide you through the testing process and discuss the results with you.