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Ins and Outs of Prenatal Genetic Testing

By Dr. Joseph Martin

One of the choices you will make as a prospective parent involves whether or not to test for genetic conditions in your developing baby. There are many choices, as well as many misconceptions about genetic testing.  

There are two types of conditions that your doctor may discuss with you: Trisomies (when the baby has an abnormal number of chromosomes) and carrier mutations (when a parent carries a less-common version of a gene, which may be passed to the baby).

Chromosome or Trisomy Testing

Most people have a total of 46 chromosomes, half from their mother and half from their father. Chromosomes carry the genetic material, or genes, from generation to generation. Trisomies occur when the sperm or egg carries an abnormal number of chromosomes, resulting in a baby having 47 chromosomes. Down syndrome is a well-known trisomy condition. People with Down syndrome have an extra chromosome number 21, so this condition is called Trisomy 21. Trisomies may affect people differently, with varying degrees of possible abnormalities.  There are genetic tests offered to help detect the risk or presence of certain trisomies.

You may be aware that a mother's age affects her risk of conceiving a child with a trisomy.  For example, the overall rate of Down Syndrome is approximately 1 in 700 births; however, as a mother’s age increases, so do her chances of delivering a baby with Down Syndrome.

For chromosome conditions, you can choose to do no test, a screening test, or a diagnostic test.

No Testing

Many prospective parents choose to decline any type of testing for chromosome conditions.  They may feel that the results of the test would not alter their approach to their pregnancy, or they may feel that finding out information about trisomies may increase their worry during their pregnancy.

If you decline any genetic testing, remember that an ultrasound is routinely done midway through the pregnancy. Ultrasounds may find clues about potential genetic conditions, but they cannot diagnose trisomies.

Screening Tests

A screening test calculates the chance that your baby has a certain trisomy.  It cannot say whether or not your baby has or does not have the condition. For example:  Jane Doe has a screening test that shows her baby has a 1:100 chance of having Trisomy 21. In this situation, her baby has a 1% chance of having the condition and a 99% chance of not having it.  

Traditional screening tests are offered at two different times during the pregnancy:

  • First Trimester Screen is a test that uses an ultrasound and a small amount of the mother's blood to test for the odds of Trisomy 21 (Down Syndrome) and Trisomy 18 (Edwards Syndrome). This test is often called a Nuchal Translucency, or NT test, and is performed at 12 - 13 weeks of pregnancy.
  • AFP-tetra, or quad screen, is a blood test done around 16 weeks.  Just like the first trimester screen, this test calculates the chance that your baby can have trisomy 21 and trisomy 18.  In addition, this test also assesses the chance that your baby can have an open neural tube defect, which is a type of malformation of your baby's spine or brain that is not usually genetic.

If you choose a screening test, you would choose one or the other; both would not be done. If someone does a first trimester screen, they can do a separate test, called an MSAFP test, to assess the risk of neural tube defects in the second trimester.

If your screening test comes back with a risk for trisomy higher than you are comfortable with, you do have the choice to then proceed with a cell-free fetal DNA test or a diagnostic test. 

Cell-Free Fetal DNA Testing

Cell-free fetal DNA testing is generally offered to higher-risk couples: pregnancies involving older mothers, couples with previous children with trisomy, or those with abnormal screening test results. This test is noninvasive and only requires a blood sample from the mother. This test assesses risk by actually detecting chromosome material for trisomy 21, trisomy 18, and trisomy 13 (Patau Syndrome).  The test is considered highly accurate, and cannot result in miscarriage.  It is also possible to test for the baby's gender with these tests. Two commercial cell-free DNA tests are the MaterniT21 test and the Harmony test.  

Diagnostic Testing

Diagnostic tests have been available for many years. Diagnostic tests include chorionic villus sampling (CVS) and amniocentesis. Both of these tests involve obtaining fetal cells to analyze chromosomes.  CVS is done at the end of the first trimester, and involves a small biopsy of the placenta. Amniocentesis is done early in the second trimester, and involves using an ultrasound-guided needle to obtain some amniotic fluid through the mother's abdominal wall. Advantages of invasive testing include very high accuracy, the ability to test for many more chromosome conditions, and very low risks of complications. There is a small risk of miscarriage with either procedure, less than 1%.

Carrier Screening

Carrier screening refers to tests that determine whether you or your partner carry an unusual form of a gene that can lead to a disorder or disease in your child. Carrier screening is available before or after you have discovered you are expecting. Unlike testing for chromosome conditions in your baby, carrier screening tests for your potential to pass a possibly harmful gene to your child.  

Most carrier tests screen for conditions that require both parents to carry the unusual gene in order for the child to have significant health issues. Having a gene from one parent makes you a carrier, meaning you can pass the gene to your child but not be affected by the condition yourself.  Your child would only be affected if he or she has unusual genes from both parents.  Carrier screening can look for common conditions such as: cystic fibrosis, thalassemia, spinal muscular atrophy, Tay-Sachs, sickle cell anemia, and Fragile X syndrome. 

Are these tests covered by my insurance plan?

Some insurance plans cover many genetic tests and carrier screens.  Some do not.  Should you consider testing, it is always wise to contact your health plan to find out your coverage.  If you cannot find out easily, ask your physician’s office if they can help you find out. 

Topics and Subtopics: Pregnancy

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