Prenatal test descriptions

Screening versus diagnostic testing

Every pregnancy has a 3-5% risk to be affected with a birth defect or genetic condition. Some pregnancies may be at higher risk due to factors such as maternal age or family history. While there is no test that can guarantee a healthy baby, there are some tests that can provide additional information. There are two types of tests available in pregnancy:

Screening tests

  • Screening tests provide additional information about if a pregnancy is at higher or lower risk for certain conditions, but do not provide a definitive yes or no answer.
  • Screening tests are typically non-invasive and do not pose a risk to the pregnancy.

Diagnostic tests

  • Diagnostic tests provide a definitive answer about whether a baby is affected with a chromosome abnormality or certain genetic conditions.
  • Diagnostic tests are invasive and carry a small risk (1% or less) for miscarriage.

Carrier screening

Carrier screening is available before or during a pregnancy to help determine if a couple has a higher risk of having a child with certain genetic conditions, such as cystic fibrosis. This test is performed by collecting a maternal and/or paternal blood sample. If both a woman and her partner are carriers for the same condition, each child has a one in four (25%) chance of being affected. Certain couples have a higher chance of being carriers for the same condition, including those of certain ethnic backgrounds, those who share a common ancestor, or those who have a personal/family history of a genetic condition.

If an increased risk is identified through carrier screening, a genetic counselor can help you understand your results and discuss your reproductive options. These options may include assisted reproductive technologies (ex. IVF), prenatal diagnosis through chorionic villus sampling (CVS) or amniocentesis, or guidance in preparing for the birth of a child with a genetic condition.

Negative (normal) carrier screening results greatly reduce a couple’s chance of having a child born with specific genetic conditions, but do not completely eliminate that risk. Carrier screening does not provide information about risk for chromosome conditions, such as Down syndrome, or adult-onset conditions, such as Alzheimer’s disease.

If you are interested in carrier screening, please speak to your healthcare provider during your scheduled appointment.

Maternal serum screening

Maternal serum screening is available during a pregnancy and includes first trimester screening, Integrated screening, and quad screening. These tests are used to determine if a pregnancy is at increased risk for certain chromosome conditions and birth defects. These tests do not screen for all chromosome problems, genetic conditions, or birth defects.

  • Performed by completing an ultrasound and collecting a maternal blood sample in the first, second, or both trimesters
  • Analyzes certain hormones in the maternal blood which are produced by the placenta and fetus
  • Screens for:
    • Trisomy 21 (Down syndrome)
    • Trisomy 18 (Edwards syndrome)
    • Neural tube defects (including spina bifida)

Screening tests determine whether or not a pregnancy is at an increased risk for a given condition. A negative (normal) screening result reduces the chance for a pregnancy to be affected, but does not completely eliminate the chance. In the case of a positive (abnormal) screening result, a genetic counselor will discuss the meaning of these results and discuss next steps. Further evaluation is typically recommended and may include a detailed (level II) ultrasound and/or amniocentesis.

Cell-free fetal DNA screening

Cell-free fetal DNA screening, also called non-invasive prenatal screening, is a test used to determine if a pregnancy is at increased chance for certain chromosome conditions. This test does not screen for all chromosome problems, genetic conditions, or birth defects. This test is not appropriate for all pregnant women. At this time, it is offered to pregnant women with certain high-risk indications.

  • Performed by collecting a maternal blood sample
  • Analyzes DNA from the placenta, which is circulating in the mother’s blood. The placenta usually has the same chromosome makeup as the baby.
  • Screens for:
    • Trisomy 21 (Down syndrome)
    • Trisomy 18 (Edwards syndrome)
    • Trisomy 13 (Patau syndrome)
    • Sex chromosome abnormalities

Screening tests determine whether or not a pregnancy is at an increased risk for a given condition. A negative (normal) screening result reduces the chance for a pregnancy to be affected, but does not completely eliminate the chance. In the case of a positive (abnormal) screening result, a genetic counselor will discuss the meaning of these results and discuss next steps. Further evaluation is typically recommended and may include a detailed (level II) ultrasound and/or amniocentesis.

Chorionic villus sampling (CVS)

Chorionic villus sampling, also known as CVS, is an invasive prenatal test that provides information about the health of a baby. Most commonly, CVS is used to determine if a baby has a chromosome problem or specific genetic condition. There is certain information about a baby that a CVS is not able to provide. A genetic counselor can discuss the risks, benefits, and limitations of this test in more detail.

When and how is CVS performed?

CVS is performed by collecting a small sample of the placenta, called the chorionic villi, which is typically genetically identical to the baby. This procedure is usually performed between 10-12 weeks gestation, which is earlier than an amniocentesis.

Who can have this test?

This test is available to all pregnant women, however, it is often performed when an increased risk for a health problem in the baby is identified based on personal/family history or ultrasound findings.

What are the risks of a CVS test?

There is a risk of complications from this procedure, including bleeding, infection, rupture of membranes, and miscarriage. The risk of a miscarriage is estimated to be less than 1 in every 100 procedures (less than 1%). This risk is higher than the risk associated with an amniocentesis. There is also a small risk of an inconclusive test result, which may require follow-up with an amniocentesis.

What are the benefits of a CVS test?

Results from testing performed on chorionic villi may provide information about the health of your baby. This information can be used in many different ways. A genetic counselor can discuss the benefits of this test taking into account your personal values and beliefs.

What to expect during this procedure?

  • While the appointment will last about an hour, the procedure takes less than 10 minutes to perform.
  • A doctor will perform the procedure using ultrasound guidance.
  • Depending on the location of the placenta, this procedure is performed through the abdomen (transabdominal) or through the cervix (transcervical).
  • The transcervical procedure is performed by inserting a thin plastic tube, called a catheter, through the vagina and cervix to reach the placenta. The transabdominal procedure is performed by inserting a needle through the abdomen and uterus and into the placenta. 
  • You may feel some pain or cramping when the doctor inserts the needle.
  • If your blood type is Rh negative, you may receive a shot of medicine called RhoGAM® to prevent Rh incompatibility.
  • Instructions will be provided about how to care for yourself following this procedure. It is recommended that you avoid lifting over 25 pounds for the next 24 hours.

CVS is different from another diagnostic prenatal test called amniocentesis.

Amniocentesis (Amnio)

Amniocentesis is an invasive, diagnostic prenatal test that provides information about the health of a baby. Most commonly, amniocentesis is used to determine if a baby has a chromosome problem, specific genetic condition, or viral infection. There is certain information about a baby that an amniocentesis is not able to provide. A genetic counselor can discuss the risks, benefits, and limitations of this test in more detail.

When and how is amnio performed?

This procedure is typically performed after 16 weeks gestation by collecting a sample of amniotic fluid that surrounds the baby.

Who can have amnio?

This test is available to all pregnant women, however, it is often performed when an increased risk for a health problem in the baby is identified based on personal/family history or ultrasound findings.

What are the risks of amnio?

There is a risk of complications from this procedure, including bleeding, infection, rupture of membranes, and miscarriage. The risk of a miscarriage is estimated to be 1 in every 400 procedures (less than 1%).

What are the benefits of amnio?

Results from testing performed on amniotic fluid may provide information about the health of your baby. This information can be used in many different ways. A genetic counselor can discuss the benefits of this test taking into account your personal values and beliefs.

What to expect during amnio

  • While the appointment will last about an hour, the procedure takes less than 5 minutes to perform.
  • A doctor will perform the procedure using ultrasound guidance.
  • The ultrasound is used to determine the safest place to insert a thin needle through the abdomen and uterus into the amniotic sac to collect the amniotic fluid.
  • You may feel some pain or cramping when the doctor inserts the needle and pressure in your lower abdomen. Some women report no pain or discomfort.
  • If your blood type is Rh negative, you may receive a shot of medicine called RhoGAM® to prevent Rh incompatibility.
  • Instructions will be provided about how to care for yourself following this procedure. It is recommended that you avoid lifting over 25 pounds for the next 24 hours.

Amniocentesis is different from another diagnostic test called chorionic villus sampling (CVS).

Last reviewed: 
January 2017

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