Prenatal Testing In-Depth: Types of Tests
Good prenatal care will help you have a healthy pregnancy and make informed decisions. Care includes tests and visits with your doctor. There are many types of prenatal tests for the mother and fetus. Different tests are used at different times in your pregnancy. Some tests are done routinely. Other tests are done only when there is an important reason to do so.
Some prenatal tests are screening tests, while others are to help make a diagnosis. A screening test gives information about the possibility that a problem exists. For example, an alpha-fetoprotein (AFP) screening test can show that there is a 1% risk of spina bifida. A diagnostic test can tell that a specific problem is present.
The different types of prenatal testing include:
- Blood (or saliva) tests
- Urine tests
- Other tests
- Ultrasound, including nuchal translucency
- Chorionic villus sampling (CVS)
- Percutaneous umbilical blood sampling (PUBS)
In a blood test, blood is drawn from your arm using a syringe. It is sent to a lab for evaluation. Blood tests check for:
Blood Type and Rh Factor
There are four major blood types: A, B, AB, and O. Each of these blood types is also classified by the types of antigens present. Antigens are proteins on the surface of blood cells, one of which is called the Rh factor. If you carry this protein, you are Rh positive. If you do not carry this protein, you are Rh negative. About 85% of people are Rh positive.
When the mother is Rh negative and the father is Rh positive, the fetus has at least a 50% chance of being Rh positive. Potential Rh incompatibility may result. The mother may develop antibodies to the Rh antigens in the baby’s blood if a small amount of the baby’s Rh-positive blood mixes with the mother’s blood. This rarely occurs in a woman’s first pregnancy, because fetal blood cells usually only gain access to the mother’s circulation late in the pregnancy or during childbirth. The woman can also become sensitized to the Rh factor during a miscarriage or ectopic pregnancy.
Symptoms and complications of Rh incompatibility only affect the fetus or newborn. The mother’s health is not affected. If the mother’s antibodies cross the placenta and attack the fetus’s blood, the fetus’s red blood cells will be broken down and fetal anemia will result. This can cause swelling of the baby’s body, brain damage, heart failure, or even death of the fetus or newborn.
Rh incompatibility can be prevented in most cases by giving mothers Rh immunoglobulin (RhIg). This prevents sensitization of an Rh-negative mother’s blood. The first injection is given at week 28 of the pregnancy. The second injection is given within 72 hours of delivery, and after miscarriage or abortion to desensitize the mother’s blood for future pregnancies.
When an Rh-negative woman develops antibodies to an Rh-positive baby, RhIg treatment does not help. Therefore, Rh incompatibility prenatal testing and preventative measures are extremely important. The fetus will be closely monitored during the pregnancy. The baby may need a blood transfusion after delivery. In severe cases, the fetus may be given a blood transfusion while in the uterus.
If you are anemic, you have low levels of iron in your blood. A serum ferritin screening test may be done to determine if you are anemic. A mild level of anemia is expected. It is normal in later pregnancy due to the mother’s expanded plasma volume. But, severe or early onset anemia can lead to extreme tiredness and increase your risk ofpreterm delivery and low birth weight. You may be prescribed iron tablets or injections.
Sexually Transmitted Diseases (STDs)
Many STDs can harm your developing baby. Doctors routinely test for chlamydia and gonorrhea, which are done by culture. Blood tests are done for:
- Hepatitis B
- Human immunodeficiency virus (HIV)
It is important to know if you have any of these conditions. There are things that you can do to help prevent the STDs from spreading to your baby.
Risks of Certain Diseases
Cystic Fibrosis (CF)
CF is a genetic disease. Both parents must be CF gene carriers for the baby to get the condition. There is no cure for CF, although treatment has advanced in recent years. CF is most likely to occur if the parents are Caucasians of Northern European descent.
Some pregnant women develop high blood sugar levels during pregnancy. This is called gestational diabetes. It is due to hormonal changes in the body that affect insulin absorption. You may be asked to take an oral glucose tolerance test around the 24th to 28th week of pregnancy. This includes a special sugar-loaded drink and repeated blood tests. Women at high risk should be screened sooner.
If you have symptoms or a history of thyroid disease, you may have blood testing done to confirm it. It is usually treated with medication to bring thyroid-stimulating hormone (TSH) back to normal levels. This reduces the risk of complications for the baby and mother.
Serum Screening Tests
At 15-20 weeks, a mother’s blood components can be tested to find out if the baby is at increased risk for spina bifida, abdominal wall defect,Down syndrome, and trisomy 18 (another type of genetic disorder). At 10-14 weeks, these blood serum tests can be combined with a special ultrasound, called nuchal translucency, to identify signs of Down syndrome, trisomy 18, and other birth defects.
Triple Screen or Multiple Marker Screening
The triple screen is a maternal blood test that measures the levels of three substances normally produced by the fetus or placenta. The test is used to identify pregnancies that have a higher-than-average risk of certain birth defects, such as Down syndrome and neural tube defects.
The triple screen can be done between the 15th and the 20th week of pregnancy. However, it is most accurate when done between the 16th and 18th week. The screening is done through a normal blood draw and poses no risk to the mother or fetus.
The triple screen test is available for all pregnant women, but is recommended for those who are 35 years or older. It may also be recommended for pregnant women with:
- Birth defect history
- History of illicit drug or harmful substance use
- Viral infection
- Radiation exposure
The three substances the triple test measures are:
- Alpha-fetoprotein (AFP)—protein normally produced by the fetus
- Estriol—estrogen produced by both the fetus and placenta
- Human chorionic gonadotropin (hCG)—hormone produced within the placenta
The triple screen is a screening test, not a diagnostic test. This means the test can only assess risk. It cannot diagnose a condition. A woman’s individual risk of having a child with birth defects is calculated using the levels of AFP, estriol, and hCG combined with the mother’s age, weight, and race.
The test results are expressed as odds. For example, “the odds this baby has Down syndrome are now greater than, less than, or equal to” what was expected based on the mother’s age and medical history.
If the triple screen shows increased risk, additional testing is done, as well as a referral to a genetic specialist. It is important to remember that the test only points to the likelihood of an outcome—it is not diagnostic. Many women with a positive screening test do not have a child with a birth defect. This is considered a false positive result.
AFP is sometimes tested alone, without hCG and estriol. Also, an additional marker can be added to the test, making the triple screen a quadruple screen. The substance is inhibin A, which is a glycoprotein. Studies have found that a greater number of markers increases the accuracy of the screening test.
In a urine test, you will be asked to collect a sample of your urine in a sterile container. The urine is sent to a laboratory for evaluation. Urine tests may be done to check for:
- Glucose—The presence of sugar in your urine indicates gestational diabetes, a type of diabetes that occurs only in pregnancy. If sugar is present in your urine, additional tests will be ordered to confirm the diagnosis.
- Protein—The presence of protein in your urine could be a sign of a urinary tract infection. Later in pregnancy, protein in the urine can signal pre-eclampsia. This condition causes a sudden rise in blood pressure and may cause excessive weight gain.
- Ketones—The presence of ketones in your urine is a sign that your body is breaking down fats instead of carbohydrates for energy. This can mean that you are not getting enough to eat or you are dehydrated. This may be present in diabetes.
- Bacteria—The presence of significant numbers of bacteria in your urine is a sign of potential urinary tract infection. You may be prescribed antibiotics to treat the infection. Group B streptococcal infection can also be identified in a urine test.
A swab of your vagina or rectum may be taken to check for signs of infection or changes in cell structure. A swab or smear may be taken for:
- Cervical dysplasia or other conditions—A cervical or Pap smear is often done at the beginning of pregnancy to check for abnormalities in cells or other conditions, such as cancer.
- Group B streptococcal infection
Group B Streptococcal Infection (GBS)
Group B streptococcus (GBS) is a bacterium that can cause life-threatening infections in newborns. Many people carry GBS but do not become ill. One of every four to five pregnant women carry GBS in the rectum or vagina. During birth, a newborn may come in contact with this bacterium.
Occasionally, babies born to a mother who carries GBS will develop signs and symptoms of the disease. Most cases occur within the first week of life and most cases are apparent a few hours after birth. GBS infection can lead to severe infection, pneumonia, and meningitis. Once diagnosed, GBS infections can be treated through IV antibiotics.
A woman can be checked for GBS during pregnancy. The test should be done late in pregnancy, during weeks 35-37. A swab of both the vagina and the rectum are taken. The swabs are sent to a lab and grown in culture.
If the result of the test is positive, then you are considered a carrier of GBS. If you test positive for GBS or are at high risk, IV antibiotics will be advised during labor and delivery.
An ultrasound uses sound waves to examine internal structures, including the developing fetus and position of the placenta. Ultrasound is a noninvasive exam that poses no risk to the mother or fetus. Depending on which trimester you are in, an ultrasound can be used for different reasons.
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Timing of Ultrasound Studies
- Confirm a pregnancy
- Estimate the gestational age
- Identify location of pregnancy—in uterus or in fallopian tubes
- Check for a neural tube defect
- Check for multiple gestations
- Check for extra fluid in the back of the neck
- Week 11-14: characteristics of potential Down syndrome
- Week 18-20: possible congenital malformations
- Check for multiple pregnancies
- Confirm gestation age and fetal growth
- Check level of amniotic fluid
- Determine the baby’s sex
- Determine the location and development of the placenta
- Check fetal well being and movement
- Identify any uterine or pelvic abnormalities of the mother
- Check the adequacy of the amount of amniotic fluid
Types of Ultrasound Studies
Early in the pregnancy, a transvaginal ultrasound may be done. During this procedure, a slender probe is inserted into your vagina. Images of the uterus and ovaries are created on a computer screen. The images are evaluated by a trained professional.
Nuchal Translucency (NT)
In the first and early second trimesters (between 11-16 weeks), you may have a nuchal translucency (NT) or nuchal fold scan done. This is a special ultrasound that can help identify Down syndrome, other chromosomal abnormalities, and heart defects. The fluid in the back of your baby’s neck is measured. These results are often combined with other blood test results to determine the risk of a condition.
During this procedure, your abdomen is coated with a gel and a technician passes a wand-like tool, called a transducer, across your abdomen. Images of the fetus are created on a computer screen and evaluated by a trained professional. When this procedure is conducted during the first trimester, a full bladder is required.
Additionally, you may have a Doppler ultrasound, which can detect moving blood cells and measure their direction and speed.
Last reviewedMarch 2014by Andrea Chisholm, MD
Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.