Over the years, genetic testing has become a more reliable way for doctors and patients to identify a variety of factors. Today, women can choose multiple types of testing during pregnancy, including simple blood tests that identify a few chromosome abnormalities and the sex of a baby and more complex diagnostic screening to identify most chromosomal abnormalities and, in some cases, other conditions.
The Facts About Prenatal Genetic Testing
Prenatal genetic tests can tell whether a baby has a genetic disorder—a type of medical condition that involves a change or defect in the typical chromosome structure, number or content.
Chromosomal abnormalities differ by what’s askew with the chromosomes. Missing chromosomes or extra chromosomes are called an aneuploidy—a trisomy when there’s an extra chromosome, and when a chromosome is missing.
Inherited disorders are also considered genetic, and they are caused by changes in our genes called mutations.
Two Types of Prenatal Tests
There are two types of prenatal genetic tests—prenatal screening tests and prenatal diagnostic tests. A prenatal screening test checks to see whether your baby is more likely to have a health condition, while a prenatal diagnostic test provides specific, definitive information about whether a baby has a genetic disorder.
Prenatal screening tests are performed several ways. The first is known as carrier screening, which uses a blood sample or a sample of cells from the inside of the cheek. This test identifies whether parents carry a gene tied to an inherited disorder. While carrier screening is often done during pregnancy, it can also be performed prior to pregnancy.
Other prenatal screening tests use a blood test and images from an ultrasound exam to determine the likelihood that the fetus has one of several conditions that include an extra or missing chromosomes, and/or a birth defect. Prenatal screening includes cell-free DNA testing, which uses a sample of the mother’s blood to screen tiny fragments of DNA for conditions such as Down syndrome.
Prenatal diagnostic tests identify a potential for a genetic disorder. They often follow a screening test and use cells that come from the baby or the placenta to collect information about certain disorders. Amniocentesis and chorionic villus sampling are two examples. This type of testing carries a small risk of miscarriage.
When Prenatal Genetic Testing Is Done
Prenatal genetic testing can be performed at any time during pregnancy, but is most commonly done during the first trimester, the second trimester, or in a combined first- and second-trimester screening.
First Trimester
A woman typically undergoes a blood test and an ultrasound between weeks 10 and 13 of pregnancy. The ultrasound, called a nuchal translucency screening, measures the thickness of the skin on the back of the baby’s neck. Abnormalities in the thickness may increase the chance of Down syndrome or defects of the heart or another body part.
Cell-free fetal DNA (cfDNA) testing is also performed during this stage of pregnancy, or any time thereafter. It screens for the chance for Down syndrome and two other chromosome abnormalities called trisomy 18 and trisomy 13. The sex of the baby can be determined with cfDNA. It is a very sensitive screening test but is not diagnostic. This test is offered to many pregnant women who desire this information and is specifically offered for women 35 years of age or older, if a defect is detected on an ultrasound, or if the mother has previously had a pregnancy with a birth defect or chromosome abnormality. Chorionic villus sampling or amniocentesis, performed during the first and second trimesters, respectively, is often recommended as a follow-up test if a genetic disorder is suspected.
A genetic diagnostic test known as chorionic villus sampling can also be performed between weeks 10 and 13, if a woman chooses. This test analyzes a sample of tissue from the placenta for possible abnormalities.
Second Trimester
During the second trimester, two other prenatal screening tests are typically performed. A “quad” blood test is done between weeks 15 and 22 to determine the risk of Down syndrome, Edwards syndrome and neural tube defects. An ultrasound between weeks 18 and 22 checks for obvious physical defects throughout the body, including the brain, spine, face, heart, kidneys, GI tract and limbs.
If a heart defect is detected during this ultrasound, which is also called an anatomy scan, further testing may be recommended. A fetal echocardiogram, for instance, is a specific type of ultrasound that focuses specifically on detailed images of the heart in utero.
Another genetic diagnostic test is performed during the second trimester. Amniocentesis is often offered to women over 35 during pregnancy, since the risk of chromosomal abnormalities increases as women get older. During this test, usually performed between weeks 15 and 20, a small amount of amniotic fluid is removed and analyzed for genetic disorders.
In some cases, test results from genetic screening tests performed in the first and second trimesters may be combined. Doing so usually provides more detailed and precise results, but also delays final testing results until the second set of tests is conducted in the second trimester.
The Pros and Cons of Prenatal Genetic Testing
Prenatal genetic testing is a personal decision. Some choose not to undergo prenatal genetic testing, while others view these screenings as an opportunity to better understand what to expect and make decisions that are best for their families. Often, the detection of a genetic disorder during pregnancy allows the family to prepare for special situations during childbirth or for the care their child may require afterward.
A genetic counselor can help you decide what’s right for your situation. Along with your doctor, a counselor can provide guidance about the benefits of genetic testing. A counselor can also help you understand the results of genetic tests and what they mean for you and your child.
You Might Be Interested In…
- 3 Things to Know About the Importance of Prenatal Care
- Pregnant? Keep Safe From Lead
- Pregnant? Here’s Why You Should Consider Breastfeeding.
