Prenatal Genetic Testing and Screenings
We are dedicated to helping you understand your family’s genetic risk factors and prenatal testing options for various fetal health concerns. Our genetic counselors expertly interprete your genetic tests and answer all of your questions so you can make the best healthcare decisions for you and your family. Please note, the availability and timing of these tests may vary by location.
How common are birth defects or genetic conditions?
Every pregnancy has a baseline of 3% to 5% risk of having a birth defect or genetic condition. Women who are over the age of 35 (advanced maternal age) or have a family history of birth defects or genetic conditions are considered to be high-risk and could require additional testing.
What are birth defects?
Birth defects, also known as congenital abnormalities or congenital anomalies, are abnormal growth changes that occur during fetal development. These changes can affect how your child’s body looks, works, or both.
Some of the most common birth defects include, but aren’t limited to:
- Cleft lip and/or cleft palate
- Bone growth abnormalities
- Congenital heart conditions (CHDs)
- Clubfoot
- Cystic fibrosis
- Down Syndrome (Trisomy 21)
- Spinal muscle atrophy
- Urinary Tract Dilation (UTD)
Did you know? SSM Health, in collaboration with Saint Louis University School of Medicine, launched the first fetal care center in Missouri in 2009. Since then SSM Health Cardinal Glennon St. Louis Fetal Care Institute has become one of the leading fetal care programs in the country, offering a variety of fetal interventions, surgeries and treatments. SSM Health offers telehealth services to connect patients with maternal fetal experts when needed.
What is prenatal genetic testing?
There are two types of prenatal genetic tests: screening tests and diagnostic tests. Each test provides different information about the likelihood your baby will inherit or develop certain birth defects or genetic conditions.
What's the difference between a screening and diagnostic test?
Screening tests:
- Provide additional information about if your baby is higher or lower risk for certain conditions.
- They do not provide a yes or no answer about specific conditions.
- Are typically non-invasive and usually involve blood draws or ultrasounds.
Diagnostic tests:
- Provide a yes or no answer about if your baby has the specific chromosomal abnormality or genetic condition you are testing for.
Common genetic screening tests include:
- Cell-free DNA screening (also called non-invasive prenatal screening or NIPS) is performed after 10 weeks gestation.
- This test looks for the risk of your baby developing Down syndrome, Trisomy 18, Trisomy 13, and sex chromosome disorders like Turner syndrome and Klinefelter syndrome.
- A second trimester screening (also known as an AFP4 or quad screen) is performed between 15 and 21 weeks. Second trimester risk levels are calculated by taking into account your age and the levels of four biochemical markers being produced during the pregnancy.
- This screening combines the results from your first and second trimester screenings to determine risk information for Down syndrome, trisomy 18 and open neural tube defects such as spina bifida.
- A carrier screening is a genetic test that identifies if you carry a gene mutation that you could pass down to your baby, including: cystic fibrosis, Tay-Sachs disease, sickle cell anemia - and about 80 other genetic conditions.
Common genetic diagnostic tests include:
- Chorionic villus sampling (CVS) - this test takes cells from the placenta between 10 and 13 weeks gestation and is used to detect chromosomal abnormalities and other genetic defects.
- Amniocentesis - performed after 15 weeks gestation, thistest analyzes the amniotic fluid surrounding the fetus and is used to detect genetic disorders like Down syndrome and spina bifida.
What to know about first trimester screening:
First trimester screening is an effective way to check for any chromosomal abnormalities, prior to the second trimester (approx. 15 weeks). This screening involves two steps:
- An ultrasound to measure an area at the back of your baby’s neck, called the nuchal translucency.
- A blood sample from a finger stick to analyze the amount of free Beta-hCG and PAPP-A.
When should I get a first trimester screening?
You must be pregnant between 11 and 13 weeks, six days counting from the first day of the most recent menstrual cycle, to receive a first trimester screening. This screening provides you an opportunity to find out early on if your baby is at an increased risk of having a chromosome problem, like Down syndrome or Trisomy 18.
Approximately 80% of all babies with Down syndrome and 90-97% of babies with Trisomy 18 will have an abnormal first trimester screening result. These results are an indicator of further testing.
If your test results show an increased risk it does not mean that your baby has a chromosome problem. It only indicates that the risk for Down syndrome, Trisomy 18, or other birth defects is increased. In most cases of an abnormality though, your baby will not have a chromosome abnormality. You may choose to have additional testing, like amniocentesis or chorionic villus sampling (CVS), which can accurately determine if a chromosome abnormality is present.
If you have additional questions about your results and testing options, speak to your health care provider. You may also request a referral to speak with a genetic counselor.
Although this is good news, first trimester screenings cannot remove the risk for Down syndrome or Trisomy 18. It does not estimate risks for other chromosome abnormalities.
All pregnancies are at risk of having an extra chromosome abnormality, like Down syndrome. However, the risk does increase as your age increases.
First trimester screening is an option for all pregnant women, regardless of age. Women who are 35 or older at delivery, or have a positive family history of chromosome abnormalities, may consider CVS or amniocentesis. These diagnostic procedures can detect chromosome abnormalities with greater than 99% certainty. However, CVS and amniocentesis are associated with a risk for pregnancy complications of 0.5-1%.
Screening tests, like first trimester screening, the Quad screen or ultrasound, can adjust the risk for a chromosome problem in the pregnancy. By themselves, they cannot rule out or confirm the presence of a chromosome problem. You may feel that the screening option is a better option because these are typically non-invasive and associated with a low risk for pregnancy complications.
Diagnostic tests, such as CVS or amniocentesis, can determine with greater than 99% accuracy if a chromosome problem is present in the pregnancy. If you are interested in knowing with certainty about the presence of a chromosome problem, you may prefer this option. However, there is a risk (less than 1%) for pregnancy complications, such as miscarriage, associated with these procedures. First trimester screenings cannot replace CVS or amniocentesis because:
- The screening test alone cannot tell you whether your baby has a problem.
- The screening estimates a risk for only two chromosome problems.
- Screenings may false reassure some patients who still have an increased risk for abnormalities due to age.
You will be offered an alpha-fetoprotein (AFP) blood test between 15-20 weeks to screen for open neural tube defects, like spina bifida. Neural tube defects occur at rate of one to two per 1,000 babies. Approximately 80% of babies diagnosed with neural tube defects are detected by the AFP blood test.
Your health care provider may recommend an ultrasound in your second trimester, at approximately 20 weeks. Although many physical abnormalities can be visualized during an ultrasound, the ability to detect problems is highly dependent the position of the baby and your gestational age and size. Ultrasounds can assess the growth and structures of your baby, but it cannot guarantee normal function.
There are some instances where first trimester screenings are not possible. This can happen if your baby is not in a position that allows the nuchal measurement to be obtained, or if the pregnancy dating is over 13 weeks, six days. If you fall into this category, you do have the option of a second trimester screening, also known as the Quad screen.