What Are Prenatal Screening Tests?
Prenatal screening tests are a set of procedures that are performed during pregnancy on expectant mothers to determine whether a baby is likely to have specific birth defects. Most of these tests are noninvasive. They’re usually performed during the first and second trimesters, although some are performed during the third.
A screening test can only provide your risk, or probability, that a particular condition exists. When the results of a screening test are positive, diagnostic tests can provide a definitive answer.
Some prenatal screening tests are routine procedures, such as glucose tolerance tests, which check for gestational diabetes. Women who have a higher risk of having a child with certain conditions are usually offered additional screening tests. For example, pregnant women who’ve lived in regions where tuberculosis is common should have a tuberculin skin test.
When Are Prenatal Screening Tests Done?
First trimester screening tests can begin as early as 10 weeks. These usually involve blood tests and an ultrasound. They test your baby’s overall development and check to see if your baby is at risk for genetic conditions, such as Down syndrome. They also check your baby for heart defects, cystic fibrosis, and other developmental problems.
Second trimester screening tests occur between 14 and 18 weeks. They can involve a blood test, which tests whether a mother is at risk for having a child with Down syndrome or neural tube defects, as well as an ultrasound.
First Trimester Screening Tests
An ultrasound uses sound waves to create an image of the baby in the uterus. The test is used to determine the size and position of your baby, confirm how far along you are in your pregnancy, and find any potential abnormalities in the structure of your baby’s growing bones and organs.
A special ultrasound called a nuchal translucency ultrasound is performed between the 11th and 14th weeks of pregnancy. This ultrasound checks the accumulation of fluid at the back of your baby’s neck. When there’s more fluid than normal, this means there’s a higher risk of Down syndrome.
During the second trimester, a more detailed ultrasound, which is often called a fetal anatomy survey, is used to evaluate the baby carefully from head to toe for any birth defects. However, not all birth defects are visible by ultrasound.
Early Blood Tests
During the first trimester, two types of blood tests called a sequential integrated screening test and a serum integrated screening are conducted. They’re used to measure the levels of certain substances in your blood, namely, pregnancy-associated plasma protein-A and a hormone called human chorionic gonadotropin. Abnormal levels of either mean there’s a higher risk of a chromosome abnormality.
At your first prenatal visit, your blood may also be tested to see if you’re immunized against rubella and to screen for syphilis, hepatitis B, and HIV.
A blood test will also be used to determine your blood type and Rh factor, which determines your Rh compatibility with your growing fetus. You can be either Rh-positive or Rh-negative. Most people are Rh-positive, but if a mother is found to be Rh-negative, her body will produce antibodies that will affect any subsequent pregnancies.
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. Incompatibility occurs if a woman is Rh-negative and her baby is Rh-positive. If the woman is Rh-negative and her baby’s positive, she’ll need the shot. If her baby is Rh-negative, she won’t.
Chorionic Villus Sampling
Chorionic villus sampling is an invasive screening test that involves taking a small piece of tissue from the placenta. It’s usually performed between the 10th and 12th weeks and is used to test for genetic abnormalities, like Down syndrome, and birth defects. There are two types. One type tests through the belly, which is called a transabdominal test, and one type tests through the cervix, which is called a transcervical test.
Testing has some side effects, like cramps or spotting. There’s also a small risk of miscarriage.
Second Trimester Screening Tests
A quad marker screening test is a blood test conducted during the second trimester. It’s done to test your blood for indications of birth defects, such as Down syndrome. It measures four of the fetal proteins.
A quad marker screening is typically offered if you start prenatal care too late to receive either the serum integrated screening or the sequential integrated screening. It has a lower detection rate for Down syndrome and other birth defects than a sequential integrated screening test or serum integrated screening test.
A glucose screening test checks for gestational diabetes. This is usually a condition that can develop during pregnancy. It’s usually temporary. Gestational diabetes can increase your potential need for a caesarean delivery because babies of mothers with gestational diabetes are usually born larger. Your child may also have low blood sugar in the days following delivery.
The test is usually performed during the second trimester. It involves drinking a sugary solution, having your blood drawn, and then checking your blood sugar levels. If you do test positive for gestational diabetes, you have a higher risk of developing diabetes within the following 10 years, and you should get the test again after the pregnancy.
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. There are several types of amniocentesis.
A genetic amniocentesis tests for genetic abnormalities, such as Down syndrome and spina bifida. A genetic amniocentesis is usually performed after week 15 of the pregnancy. It may be considered if:
- a prenatal screening test showed abnormal results
- you had a chromosomal abnormality during a previous pregnancy
- you’re 35 or older
- you have a family history of a specific genetic disorder
- you or your partner is a known carrier of a genetic disorder
A maturity amniocentesis is performed later, during the third trimester, to determine whether or not your baby’s lungs are ready for birth. This diagnostic test is only done if a planned early delivery through either induction of labor or a cesarean delivery is being considered for medical reasons. It’s usually done between weeks 32 and 39.
Third Trimester Screening Tests
Group B Strep Screening
Group B Streptococcus (GBS) is a type of bacteria that can cause serious infections in pregnant women and newborns. GBS is often found in the following areas in healthy women:
- the mouth
- the throat
- the lower intestinal tract
- the vagina
GBS in the vagina generally isn’t harmful to a woman regardless of whether she’s pregnant. However, it can be very harmful to a newborn baby who doesn’t yet have a robust immune system. GBS can cause serious infections in babies exposed during birth. You can be screened for GBS with a swab taken from your vagina and rectum at 35-37 weeks.
If you test positive for GBS, you’ll receive antibiotics while you’re in labor to reduce your baby’s risk of contracting a GBS infection.
Prenatal Diagnostic Tests
Diagnostic tests are the next step after a screening test that yields a positive result. They leave virtually no doubt as to the existence or absence of certain birth defects.
Diagnostic tests are more invasive than screening tests. They’re usually only done if there’s a specific concern, if a screening test indicates a problem, or if your age, family history, or medical history suggests that you may have an increased risk for a problematic pregnancy. Diagnostic tests can carry a very small risk of miscarriage.
Talk to Your Doctor
Prenatal screening tests can be an important source of information for many people. While many of these tests are routine, some can be a much more personal decision. Talk to your doctor about your concerns if you’re not sure if you should be screened or if you’re feeling anxious. You can also ask to be referred to a genetic counselor. Your healthcare team can discuss the risks and benefits with you and help you decide whether prenatal screening is right for you.