Prenatal Screening Tests

Written by Tracy Stickler | Published on March 15, 2012
Medically Reviewed by Jennifer Gunter, MD, OB/GYN

Overview

Prenatal screening tests are safe, non-invasive procedures that identify small subgroups of expectant mothers that may be more likely to have a baby with a particular birth defect. However, for each individual pregnancy, a screening test can only provide a risk or probability that a particular condition exists. Women found to be at increased risk based on screening would then be offered a diagnostic test for a definitive answer. 

Women who are at a higher risk for certain conditions will undergo further screening tests. For example, pregnant women who have lived in regions where tuberculosis is common should have a tuberculin skin test; all women will undergo a glucose tolerance test to check for gestational diabetes; and women who are at risk for ectopic pregnancy or miscarriage should have their blood evaluated for levels of beta-hCG hormone.

There are a variety of prenatal screening tests to help identify if a pregnant woman is at an increased risk of carrying a baby with certain birth defects. There are three different kinds of screening tests. Testing can begin as early as 10 weeks, and involves one or two blood tests and possibly an ultrasound depending on the specific testing method chosen.

Ultrasound

This screening test uses sound waves to create an image of the baby in the uterus. The test is used to determine the size and position of the fetus, as well as any potential abnormalities in the structure of the growing bones and organs of the baby. A special ultrasound called a nuchal translucency ultrasound is performed between 11 weeks and 2 days and 14 weeks and 2 days. Nuchal translucency refers to the accumulation of fluid at the back of the baby’s neck. Nuchal translucency may be increased in cases of Down syndrome and other types of birth defects, particularly major heart defects. 

Ultrasound can also be used in the first trimester to confirm how far along a woman is in her pregnancy and at 17-20 weeks a more detailed ultrasound (often called a “fetal anatomy survey” or level 1 ultrasound) is used to carefully evaluate the baby from head to toe for any birth defects, although not all birth defects are visible by ultrasound. A level 2 ultrasound is performed if a level 1 ultrasound is abnormal, or if the prenatal screening tests are abnormal.

Blood Test

At your first prenatal visit you will have a blood test to ensure you are immunized against rubella and to screen for three infections: syphilis, hepatitis B, and HIV.

A blood test will also be used to determine the mother’s blood type and Rh factor. Everyone inherits blood cells that are either Rh positive, or Rh negative. The blood test will be used to determine the pregnant woman’s Rh compatibility with her growing fetus. When there’s an Rh incompatibility, most women will be given a shot of Rh-immune globulin at 28 weeks and again a few days after delivery. This shot prevents a woman from developing antobidies to her baby’s Rh positive blood cells.

Prenatal Screening Tests

There are three different blood tests that are offered as part of prenatal screening for birth defects. The blood tests look at proteins produced by the baby and measure the levels of these proteins in the mother’s blood. There are a total of six fetal proteins that can be tested. These tests identify babies with Down Syndrome; other genetic conditions such as Trisomy 18, and babies with a condition called an open neural tube defect. An open neural tube defect is also known as spina bifida and is a condition where the spinal cord/and or brain does not form correctly very early in the pregnancy.

The sequential integrated screen measures all six of the fetal proteins found in the mother’s blood. The first blood test is performed between 10 and 13 weeks 6 days. The second blood test is performed between 15 and 20 weeks. A nuchal translucency ultrasound is also performed between 11 weeks 2 days and 14 weeks 2 days. This test is the most thorough and will identify approximately 92 percent of babies with Down Syndrome.

The serum integrated screen is the blood testing of the six fetal proteins just as described for the sequential integrated screen (first blood test between 10 weeks and 13 weeks 6 days and second blood tests between 15 and 20 weeks). The serum integrated screen identifies approximately 88 percent of babies with Down Syndrome. A serum integrated screen is typically the choice in centers where nuchal translucency ultrasound is not available (this special first trimester ultrasound requires specific training).

The quad marker screen is actually the second blood test of the sequential (and serum) integrated screen, meaning it is performed between 15 and 20 weeks. It measures four of the fetal proteins. A quad marker screen is typically offered when a women starts prenatal care too late to receive either the serum integrated screen or the sequential integrated screen. It has a lower detection rate for Down Syndrome and other birth defects than the other two tests, but still detects 79 percent of pregnancies with Down Syndrome.

Glucose Screening

A glucose screening test checks for gestational diabetes, which is a (usually) temporary condition that can develop during pregnancy. Gestational diabetes can increase the potential need for a cesarian section (because it can lead to the growth of very large babies), and can cause problems in the infant, such as low blood sugar. The test involves drinking a sugary solution, having your blood drawn, and then checking your blood sugar levels. Glucose screenings are usually performed during the second trimester. If you do test positive for gestational diabetes you are also at higher risk for diabetes later in life and should be retested after the pregnancy.

Group B Strep Screening

Group B streptococcus (GBS) is a type of bacteria that can cause serious infections in pregnant women and newborns. GBS (part of the normal bacteria found in healthy individuals) is often found in the mouth and throat, lower-intestinal tract, and vagina of healthy women. While GBS in the vagina is generally innocuous to a woman (whether she is pregnant or not), it can be very harmful to a newborn baby who does not yet have a robust immune system. GBS can cause serious infections for babies exposed during birth; women are screened with a swab taken from the vagina and rectum at 35-37 weeks, and women who test positive for GBS will receive antibiotics in labor to reduce their baby’s risk of contracting a GBS infection.

Prenatal Diagnostic Tests

Diagnostic tests examine individual pregnancies and leave virtually no doubt as to the existence or absence of certain birth defects. Diagnostic tests are more invasive than screening tests, and are usually only done if there is a specific concern if a screening test indicates a problem, or if your age, family history, or medical history indicates an increased risk for a problematic pregnancy. Diagnostic tests are also performed if a screening test is positive (remember, screening tests don’t identify birth defects, they indicate if a pregnancy is at higher than expected risk for a birth defect). Diagnostic tests can carry a very small risk of miscarriage. 

Amniocentesis

During amniocentesis, amniotic fluid is removed from the uterus for testing. Amniotic fluid surrounds the baby during pregnancy; it contains fetal cells with the same genetic makeup as the baby, as well as various chemicals produced by the baby’s body. This diagnostic test is used in a number of ways:

Genetic Amniocentesis

Genetic amniocentesis tests for genetic abnormalities such as Down syndrome and spina bifida. Genetic amniocentesis is typically performed after week 15 of the pregnancy, and may be considered if:

  • A prenatal screening test showed abnormal results.
  • You had a chromosomal abnormality during a previous pregnancy.
  • You are 35 or older.
  • You have a family history of a specific genetic disorder, or you or your partner is a known carrier of a genetic disorder.

Maturity Amniocentesis

Maturity amniocentesis is performed to determine whether or not a baby’s lungs are ready for birth. This diagnostic test is only done if a planned early delivery (either an induction of labor or a cesarean) is being considered for medical reasons. It is typically done between weeks 32 and 39.

Other Types of Amniocentesis

Although all of these types of amniocentesis are very rare, the procedure may also be performed to check a baby for infection, to decrease the volume of amniotic fluid, to diagnose a uterine infection, or to evaluate the seriousness of an Rh incompatibility between mother and fetus.

Chorionic Villus Sampling

Chorionic villus sampling (CVS) is a prenatal test used to check for genetic disorders in the fetus such as Tay-Sachs disease, cystic fibrosis, and chromosomal abnormalities like Down syndrome. A CVS test will not reveal all birth defects. During CVS, a sample of chorionic villi is removed from the placenta, and is then used to test the DNA, chromosomes, and enzymes of the fetus. The chorionic villi make up most of the placenta, and share the baby’s genetic makeup. A CVS test is typically performed during weeks 10 to 12, and may be considered when:

  • A prenatal screening test showed abnormal results.
  • You had a chromosomal abnormality during a previous pregnancy.
  • You are 35 or older.
  • You have a family history of a specific genetic disorder, or you or your partner is a known carrier of a genetic disorder.
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