Prenatal Tests

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What, When and Why

If you’re expecting a baby, expect to undergo a variety of medical tests during pregnancy. These are done to check for potential health problems that could affect you or the baby developing in your womb. Some are only necessary if there is a risk of genetic conditions or birth defects.

Talk with your healthcare provider about any questions you have related to your pregnancy and testing you may need.

Here’s a look at some common prenatal tests and when they might be conducted during pregnancy:

First Trimester

Blood Type and Rh Factor Test

A simple blood test, typically done during your first prenatal care visit, will let your healthcare provider know your blood type and Rh factor. The Rh factor—a protein found on red blood cells in most people—is important in pregnancy. If you have don’t have this protein, but your fetus does, it can cause health problems in your baby. Learn more about this and how any resulting problems are treated.

Noninvasive Prenatal Screening

A test of your blood to check your baby’s DNA for certain genetic conditions, including Down syndrome. Recommended if you had an earlier pregnancy involving birth defects or after abnormal first trimester screening results (see below). NOT recommended if there’s no risk of birth defects or if you’re pregnant with multiples. Typically given after 10 weeks.

First Trimester Screening

A blood test and an ultrasound to check for possible birth defects, such as Down syndrome and heart defects. At 11–13 weeks of pregnancy.

Chorionic villus sampling (CVS)

Tiny tissue samples from your placenta are extracted by a needle inserted through your abdominal wall or by a thin tube (catheter) inserted vaginally up through your cervix. The samples, which have the same genetic makeup as your baby, are then tested for genetic abnormalities, such as Down syndrome. At 10–12 weeks.

CVS is an invasive procedure with a small risk of infection or miscarriage. The test is only recommended if your baby has a higher risk of birth defects because you:

  • Are 35 or older
  • Tested positive in a noninvasive prenatal screening test
  • Had an ultrasound that indicated possible abnormalities
  • Had a previous pregnancy involving a genetic condition (such as Down syndrome)
  • Have a family history of a specific genetic condition

Carrier screening

These tests, which can be done anytime in pregnancy (or before), look at whether you or your partner carry a gene for certain inherited disorders, called recessive disorders. Each parent needs to carry a gene for the particular recessive disorder (though not necessarily be affected by it) in order for the baby to have the disorder. These recessive disorders include cystic fibrosis, spinal muscular atrophy, sickle cell disease and other.

Second Trimesters

Maternal blood screening (or quad screen)

Blood test to look again for potential birth defects in your baby. This test measures 4 substances in your blood. At 15–20 weeks.


A needle inserted through your abdominal wall withdraws a small sample of amniotic fluid to test for genetic conditions, such as Down syndrome and a neural tube defect (a serious condition affecting your baby’s brain or spine).

Like a CVS, amniocentesis is only recommended if you have certain risk factors, including being over age 35; having a family history of—or a previous pregnancy involving—a genetic condition; or abnormalities picked up in an ultrasound or earlier screening tests. At 15–20 weeks.


Ultrasounds use sound waves to create an image of your baby on a video screen. Usually performed at least once during most pregnancies, an ultrasound can help estimate a due date, show the progress of a baby’s development or reveal if you’re having a single baby or twins. It can also usually tell you the baby’s gender.

After the sixth month, an ultrasound is used to monitor the size and position of the baby, the volume of amniotic fluid and the position of the placenta. Ultrasounds are painless, although in early pregnancy they require a full bladder to help lift the uterus into view. At 18–20 weeks.

Glucose screening

A blood test that checks for signs of gestational diabetes, a type of diabetes that some women develop during pregnancy. If you undergo this test, you will be asked to drink a concentrated sugar solution. After a short period, a blood sample will be drawn and tested to analyze how well your body reacts to the sugar.

If these test results are high, a three-hour glucose test is done to make the diagnosis. Caught early, gestational diabetes can usually be managed through diet and exercise. If you’ve had a standard gastric bypass procedure as opposed to a Lap Band your provider may want to use an alternative way to screen for diabetes. At 24–28 weeks.

Third Trimester

Antibody Screen

If an earlier blood test found that you have Rh-negative blood and your fetus has Rh-positive blood, your blood types are considered incompatible. At week 28, you may need another blood test to detect whether your body is forming antibodies against your fetus’s red blood cells. If no antibodies are found, you may receive an injection of a special blood product to help prevent them from forming. Learn more about the Rh factor in pregnancy and why it has to be managed.

Group B strep (GBS) test

Fluid extracted from your cervix is tested for signs of GBS, a type of strep that can be passed along to your baby during childbirth. At 37–38 weeks.

Late Pregnancy

If your labor has not yet begun, your healthcare provider may want you to have a few tests that will monitor the well-being of your baby as your pregnancy continues.

Non-Stress Test

A non-stress test is the simplest and most common. It’s based on the observation that the heart rate of a healthy baby accelerates by about 15 beats per minute for 15 seconds when the baby moves.

During the test, your caregiver will try to get your baby moving a little (with sound or by giving you a high-glucose snack) and then count his heartbeats.

Sometimes a baby is sleeping during the test and doesn’t react. If that happens, the test will just be repeated later in the day.

Contraction Stress Test (CST)

A contraction stress test (CST) is a less common test in late pregnancy. With a CST, gentle labor contractions are encouraged by stimulating your nipples or by giving you a little of the labor-stimulating drug Pitocin. The CST test indicates if a baby is stressed by labor contractions.

We may also ask you to keep count of how many times your baby moves within a given time period.

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