Prenatal Genetic Testing

Prenatal genetic testing gives parents information about whether their baby has certain genetic disorders. Genetic disorders are caused by changes in a person's genes or chromosomes, the basic map of the human body.

There are two main types of prenatal genetic tests:

• Prenatal screening tests: these tell you the chances that your baby has a genetic problem.
• Prenatal diagnostic tests: these tell you whether your baby actually has a certain disorder.

Screening tests include carrier screens and prenatal genetic screens.  

• Carrier screening is done on parents using a blood sample or mouth swab. These tests are used to find out whether a person carries a gene for certain inherited disorders. They can be done before or during a pregnancy.  For example, screening for sickle cell disease, cystic fibrosis or Tay-Sachs disease.
• Prenatal genetic screening done on the pregnant mother by a blood sample or by ultrasound of the baby.  These can screen for genetic defects and birth deformities.  Birth deformities may exist as a syndrome with a genetic link or on its own.

First trimester screening includes a test of the pregnant woman's blood and an ultrasound. Both are done together between 10-13 weeks in the pregnancy.

• A blood sample measures the level of two substances.
• The ultrasound exam, called the nuchal translucency screening, measures the thickness of a space at the back of the baby's neck. An abnormal measurement means there is an increased risk that the baby has Down syndrome or another type of chromosomal problem.

Second trimester screening includes a test of the mother's blood and an ultrasound. These are done between 15-22 weeks of the pregnancy.

• The "quad" or "tetra" blood test measures the levels of 4 different substances in the blood and calculates a risk of Down syndrome, trisomy 18, and neural tube defects.
• An ultrasound exam done between 18-22 weeks checks for major physical defects in the brain, spine, facial features, abdomen, heart and limbs.

Combined first and second trimester screening, also called sequential or integrated screening, is an option for screening for genetic problems.  This test will combine the Nuchal translucency testing of the first trimester with the Quad/Tetra screens of the second to give a composite risk. It is a more accurate results to assess risk.

Cell-free DNA testing can be done by finding the small amount of DNA that is released from the placenta into a pregnant woman's bloodstream.  This cell-free DNA can be screened for Down syndrome, trisomy 18, trisomy 13 and the sex chromosomes. It can be done from 10 weeks on in the pregnancy.  This will work best in women who already have an increased risk of having a baby with a chromosome disorder.  For women with a low risk of having a baby with a chromosome disorder, the first trimester nuchal translucency, tetra or combined screening are the recommended testing.

Talk with your doctor about which testing is best for you and your pregnancy.

Considerations about Prenatal Genetic Testing at Kelsey Seybold Clinic

It is your choice whether to have prenatal testing.  Your personal beliefs and values are important factors in the decision about prenatal testing. It can be helpful to think about how you would use the results of prenatal screening test in your pregnancy care. Remember that a positive screening test tells you only that are at higher risk of having a baby with Down syndrome or another genetic problem.  A diagnostic test should be done if you want to know a more certain result.  Some parents want to know beforehand that their baby will be born with a genetic disorder.  This knowledge gives parents time to learn about the disorder and plan for the medical care that the child may need. Some parents may decide to end the pregnancy in certain situations. Other parents do not want to know this information before the child is born. In this case, you may decide not to have follow up diagnostic testing if a screening test result is positive. Or you may decide not to have any testing at all.  There is no right nor wrong answer.

Most of the time, the results of a screening test are negative (normal). A negative result does not rule out the possibility that the fetus has a genetic disorder. It only tells you that the fetus does not have the particular disorder that was tested for. If a test is positive, your doctor will explain the results and provide guidance about your choices, options, and recommended next steps that is best for you and your family.