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by Neff DM

Good prenatal care will help you have a healthy pregnancy and make informed decisions. Care includes visits with your doctor and tests. 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 is done if there is a risk of a problem. A diagnostic test can tell if a specific problem is present.

The different types of prenatal testing are:

Prenatal Testing

Blood (or Saliva) Tests

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 4 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 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.

When the mother is Rh negative and the father is Rh positive, the fetus has at least a 50% chance of being Rh positive. This can lead to Rh incompatibility. If the baby's blood enters the mother, it will change her immune system. The system will see and label the Rh factor as foreign and label it for attack. Antibodies against Rh will be made by the mother's body and remain after pregnancy. A mother will usually not be exposed to blood from the fetus until late in pregnancy or childbirth. This is why Rh incompatibility is rare in first pregnancies. Any pregnancy after will be at risk. Miscarriage or ectopic pregnancies can also introduce Rh to the mother's body.

Rh incompatibility only affects the fetus or newborn. The mother’s antibodies can pass to the fetus and attack the blood. Red blood cells will be damaged. This can lead to fetal anemia. The fetus will not get enough oxygen to the body. It can cause swelling of the baby’s body, brain damage, heart failure, or even death.

A treatment is available. The mother can receive medicine that stops her body from making antibodies. The first injection is given at week 28 of the pregnancy. The second injection is given within 72 hours of delivery to make future pregnancies safer. It may also be given after miscarriage or abortion to make future pregnancies safer.

The treatment does not help women who already have antibodies. This is why Rh incompatibility prenatal testing and preventive measures are important. The fetus will be closely watched during the pregnancy. The baby may need a blood transfusion after delivery. It will give the baby healthy red blood cells. In people with severe symptoms, the fetus may be given a blood transfusion while in the uterus.

Iron-deficiency Anemia

Anemia is a low level of red blood cells. One form of anemia is due to low levels of iron in your blood. A serum ferritin screening test may be done to check for anemia. A mild level of anemia is common in pregnancy. It is normal in later pregnancy because there is more plasma. The extra plasma "waters down" blood volume. Severe or early onset anemia will need treatment. It will cause extreme tiredness and raise the risk of preterm delivery and low birth weight. You may be given iron tablets or injections.

Sexually Transmitted Diseases (STDs)

Many STDs can harm your growing baby. Doctors routinely test for chlamydia and gonorrhea. This is done by culture. Blood tests are done for:

  • Hepatitis B
  • Syphilis
  • Human immunodeficiency virus (HIV)

It is important to know if you have any of these problems. There are things that you can do to help prevent the STDs from spreading to your baby.

Risks of Certain Diseases

Chickenpox and German measles are usually mild diseases in people who are not pregnant. However, they can cause birth defects if a mother is infected for the first time during pregnancy.

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, but early treatment can improve outcomes. CF is most likely to happen if the parents are of Northern European descent.

Gestational Diabetes

Some mothers develop high blood sugar levels during pregnancy. This is called gestational diabetes. It is due to hormone changes that affect how the body uses insulin. 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. People who are high risk should be screened sooner.

Thyroid Disease

Thyroid tests may be done if you have symptoms or a history of thyroid disease. It is done with simple blood tests. The thyroid makes a number of hormones. Problems with these hormone levels can affect the mother and baby. It can be treated with medicine to lower or replace the hormones.

Serum Screening Tests

At 15 to 20 weeks, a mother’s blood components can be tested to find out if the baby is at higher risk for spina bifida, abdominal wall defect, Down syndrome, and trisomy 18 (another type of genetic disorder). At 10 to 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 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 advised for those who are 35 years or older. It may also be advised for those with:

  • Birth defect history
  • History of substance use disorder
  • Diabetes
  • 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 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 health history.

If the triple screen shows increased risk, more 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. A greater number of markers raises the accuracy of the screening test.

Urine Tests

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 points to gestational diabetes, a type of diabetes that happens only in pregnancy. If sugar is present in your urine, more 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 point to 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 a potential urinary tract infection. You may be given antibiotics to treat the infection. Group B streptococcal infection can also be identified in a urine test.

Other Tests

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 deadly infections in newborns. Many people carry GBS but do not get sick. 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.

Sometimes, a baby born to a mother who carries GBS will develop signs and symptoms of the disease. Most times, this happens within the first week of life and is noticed a few hours after birth. GBS infection can lead to severe infection, pneumonia, and meningitis. Once diagnosed, GBS infections can be treated with antibiotics.

A woman can be checked for GBS during pregnancy. The test should be done late in pregnancy, during weeks 35 to 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, antibiotics will be given during labor and delivery.

Ultrasound

An ultrasound uses sound waves to examine internal structures, including the growing 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.

Ultrasound
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Timing of Ultrasound Studies

First trimester:

  • Confirm a pregnancy
  • Estimate the gestational age
  • Identify the location of a 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

Second trimester:

  • Week 11 to 14: signs of potential Down syndrome
  • Week 18 to 20: possible congenital malformations
  • Check for multiple pregnancies
  • Confirm gestation age and fetal growth
  • Check level of amniotic fluid
  • Determine the baby’s sex

Third trimester:

  • 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 levels of amniotic fluid
Types of Ultrasound Studies
Transvaginal Ultrasound

Early in the pregnancy, a transvaginal ultrasound may be done. During this procedure, a probe is inserted into your vagina. Images of the uterus and ovaries are created on a monitor. The images are evaluated by a trained professional.

Nuchal Translucency (NT)

In the first and early second trimesters (between 11 and 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.

Transabdominal Ultrasound

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.

Doppler Ultrasound

A Doppler ultrasound is done to detect moving blood cells and measure their direction and speed.

Additional Prenatal Testing for Genetic Disorders

A genetic disorder can be passed from one or both parents to the baby. Examples are:

The decision to have genetic testing is a complex and often difficult one. Genetic counseling is available to mothers and fathers. The following tests may be done to diagnose or confirm the presence or absence of a genetic disorder. The tests may be combined with or follow other tests results, such as blood or ultrasound.

Chorionic Villus Sampling (CVS)

Chorionic villus sampling (CVS) is a diagnostic procedure during which a sample of chorionic villi cells from the placenta is removed and tested for chromosomal abnormalities and genetic birth defects. CVS is done at 10 to 12 weeks gestation and can test for diseases such as Down syndrome, Tay Sachs, and cystic fibrosis. CVS can also be used for paternity testing.

CVS Techniques
  • Transcervical (TC) CVS—A thin tube is guided through the vagina and cervix and into the placenta where a small amount of tissue is taken. Ultrasound is used to help guide the doctor and lower the risk to the fetus.
  • Transabdominal (TA) CVS—Similar to amniocentesis, a needle is guided from the abdomen and into the placenta where a small amount of tissue is taken. A local anesthetic may be used. Just as with TC CVS, ultrasound is used to help guide the doctor and lower the risk to the fetus.

Amniocentesis

An amniocentesis is a diagnostic procedure during which a sample of amniotic fluid is removed and evaluated. Amniotic fluid surrounds a developing fetus. It contains cells normally shed from the fetus.

Amniocentesis
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When done early in pregnancy (15th to 18th week), it is used to detect:

  • Down syndrome
  • Other chromosomal abnormalities
  • Neural tube defects, such as spina bifida and anencephaly
  • Inherited metabolic disorders

Percutaneous Umbilical Blood Sampling (PUBS)

Percutaneous umbilical blood sampling (PUBS) is a diagnostic procedure that examines blood from the fetus. The blood is drawn from the umbilical cord where it inserts into the placenta. Performed at 18 weeks or later, PUBS is used to detect:

  • Chromosomal abnormalities, such as Down syndrome
  • Blood disorders, such as fetal hemolytic disease
  • Malformations of the fetus
  • Fetal infection
  • Fetal anemia

PUBS can also be used as a technique to transfuse a fetus that has fetal isoimmunization (when an Rh-negative mother produces antibodies to Rh factor) or fetal infection with parvovirus B19. This type of treatment is rarely needed now because severe fetal anemia from isoimmunization is largely prevented by other measures. Additionally, PUBS can be used to give certain medicines to the fetus.

References

Cordocentesis: Percutaneous umbilical blood sampling. American Pregnancy Association website. Available at: http://www.americanpregnancy.org/prenataltesting/cordocentesis.html. Accessed August 27, 2021.

Genetic disorders. American College of Obstetricians and Gynecologists. Available at: https://www.acog.org/womens-health/faqs/genetic-disorders. Accessed August 27, 2021.

Getting a urinalysis: about urine tests. American Pregnancy Association website. Available at: https://americanpregnancy.org/prenatal-testing/urine-test-urinalysis. Accessed August 27, 2021.

Prenatal patient support and delivery considerations. EBSCO DynaMed website. Available at: https://www.dynamed.com/management/prenatal-patient-support-and-delivery-considerations. Accessed August 27, 2021.

Prenatal ultrasound screening. EBSCO DynaMed website. Available at: https://www.dynamed.com/evaluation/prenatal-ultrasound-screening. Accessed August 27, 2021.

Routine prenatal care. EBSCO DynaMed website. Available at: https://www.dynamed.com/management/routine-prenatal-care-36. Accessed August 27, 2021.

Rh factor. American Pregnancy Association website. Available at: https://americanpregnancy.org/healthy-pregnancy/pregnancy-complications/rh-factor. Accessed August 27, 2021.

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