Prenatal Testing

Information on Prenatal Testing from WebMD

Your Guide to Prenatal Testing

When you’re pregnant, prenatal tests give you information about your health and your baby’s. They help detect any problems that could affect him, like birth defects or genetic diseases. The results can help you make the best health care decisions before and after your child is born.
Prenatal tests are helpful, but it’s important to know how to interpret what they find. A positive test result doesn’t always mean your baby will be born with a disorder. You’ll want to talk with your doctor, midwife, or other health care provider about what the tests mean and what you should do once you have the results.
Doctors recommend some prenatal tests for all pregnant women. Only some women will need other screening tests to check for certain genetic problems.

Routine Prenatal Tests

First-trimester tests

Prenatal tests are one of the many ways your healthcare practitioner will check on the well-being of you and your growing baby.
At your first prenatal visit, your practitioner will give you a thorough physical, including a pelvic exam. You'll have a Pap smear (unless you've had one recently) to check for abnormal cells. You may also have a culture to check for chlamydia and gonorrhea.
Some women will have an ultrasound in their provider's office during their first prenatal visit to confirm the pregnancy and the baby's due date.
Next, you'll have routine blood tests to identify your blood type and Rh status, as well as a blood count to check for anemia.
The lab will also test your blood for:
Finally, you should be offered a blood test for HIV. HIV stands for human immunodeficiency virus, which is the virus that causes acquired immune deficiency syndrome (AIDS). If your practitioner doesn't offer you an HIV test, be sure to ask about it. If you test positive for HIV, being treated during pregnancy can dramatically reduce your chances of passing the infection to your baby.

In addition to taking a sample of your blood, your practitioner will ask for a urine sample to test for urinary tract infections and other conditions.

If you're at high risk for gestational diabetes, a glucose challenge test might be done at your first visit.
In some cases, your provider will also do a skin test to see if you've been exposed to tuberculosis.

Second-trimester tests

During each of your second-trimester prenatal visits, your practitioner may ask you for a urine sample to screen for signs of preeclampsia, urinary tract infections, and other conditions.
Most practitioners routinely order an ultrasound between 16 and 20 weeks to check for physical abnormalities.

Between 24 and 28 weeks, you'll be given a glucose screening test to check for gestational diabetes, and possibly another blood test to check for anemia.

If you're Rh-negative but your baby's father isn't (or you don't know whether he is), an extra tube of blood may be drawn to check for Rh antibodies before you're given an injection of Rh immune globulin at 28 weeks.

Third-trimester tests

During your third-trimester prenatal visits, your practitioner may continue to ask for urine samples to check for signs of preeclampsia, urinary tract infections, and other conditions.

Between 35 and 37 weeks, you'll be tested for a common infection called group B strep. If your test is positive, you'll be given antibiotics during labor to help keep you from passing the bacteria on to your baby.

(If you've had a group B strep urinary tract infection during this pregnancy or have previously given birth to a baby infected with group B strep, you won't need testing because you'll automatically be treated during labor.)

Here are some other tests you may have this trimester:
  • If your blood glucose level was elevated when you took your glucose challenge test, you'll have a glucose tolerance test to determine whether you have gestational diabetes.
  • Your blood may be checked again for anemia, particularly if it wasn't retested late in the second trimester or if you were anemic earlier in your pregnancy.
  • If you're at risk for sexually transmitted infections (STIs), you'll be tested again for syphilis, chlamydia, gonorrhea, and HIV.
  • If you were found to have placenta previa or a low-lying placenta during an earlier ultrasound, you'll have another ultrasound to check the location of your placenta.
If your pregnancy is high risk or your practitioner becomes concerned about certain problems, you may have additional ultrasounds, a biophysical profile or a nonstress test to make sure your baby's thriving.

When and how often you go for these tests will depend on your caregiver's reason for testing. If there's any concern about your baby's growth, you'll have periodic ultrasounds to measure him and check your amniotic fluid level.

If your pregnancy is normal but you go past your due date, you'll have testing to make sure your baby's still doing well. Between 40 and 41 weeks, you may get a full biophysical profile or a modified one, which would include a nonstress test to assess your baby's heart rate and an ultrasound to check your amniotic fluid level. These tests are usually performed twice a week to help your practitioner decide whether it's safe to continue waiting for labor to start on its own.

Prenatal Genetic Tests

Doctors also can use prenatal tests to look for signs that your baby is at risk for certain genetic disorders or birth defects. You don’t have to have these tests, but your doctor may suggest some to make sure your baby is healthy.

They’re especially important for women who have a higher risk of having a baby with a birth defect or a genetic problem.This is you if you:
Some prenatal genetic tests are screening tests. They tell you if your baby has a higher risk of having a certain disorder or disease, but they can’t tell you for certain that he’ll be born with it. Other “diagnostic” tests will give you a more definite answer. Usually, you’ll get this kind after you have a positive result on a screening test.

To start, your doctor may want to test you and the child’s other parent for the genes that cause certain genetic diseases, like cystic fibrosis, Tay-Sachs disease, sickle cell disease, and others. If both of you have a gene for one of these diseases in your DNA, you could pass it on to your baby, even if you don't have the disease itself. The exam is called a carrier test.
Your doctor can use one or more different screening tests to check your baby for a genetic problem, including:
Ultrasound. You’ll already have one of these early in your pregnancy to make sure everything is going well. But if you have a high-risk pregnancy, you’ll need this exam more often. Around 11-14 weeks, doctors can use it to look at the back of your baby’s neck. Folds or thick skin there could mean a higher risk of Down syndrome. Your doctor also may take a sample of your blood at the same time.

Integrated Screening. There are two phases to this test. In the first part, doctors combine the results of the ultrasound looking at your baby’s neck and the blood tests you got at 11-14 weeks. Then, they’ll take a second blood sample between 16-18 weeks. The results measure your baby’s risk for Down syndrome and spina bifida, a spinal cord and brain disorder.

 Sequential Screen. This is similar to integrated screening, but your doctor reviews the results with you right after the first phase at 11-14 weeks. It’s not as accurate as the longer test, but it lets you know your baby’s risk earlier. If the screening finds there may be a problem, your doctor will use more tests to find out for sure. If it doesn’t find a risk, you’ll most likely get the second blood test at 16-18 weeks to be safe.

Triple or quadruple screening test. Doctors check your blood for hormones and proteins that come from your baby or your placenta, the organ that brings him oxygen and nutrients. The test can look for three different substances (triple screening) or four (quadruple screening). Certain amounts of these mean your baby has a higher chance of having a birth defect or a genetic disease. This test happens in the second trimester, usually at 15 to 20 weeks.

Cell-free fetal DNA testing. Doctors use this test to find your baby’s DNA in your blood and check it for Down syndrome and two other genetic conditions, trisomy 18 and trisomy 13. You can have this done after 10 weeks of your pregnancy. Doctors don’t recommend it for every woman, usually only those who have a high-risk pregnancy. It’s not available everywhere, and some health insurance policies don’t cover it. Talk to your doctor about whether you need this test.

Other Tests

If you get a positive result on a screening, your doctor can use other tests to look for a problem.
Amniocentesis. Using a thin needle in your belly, your doctor will take a sample of the fluid that surrounds your baby and check it for genetic disorders or birth defects. The procedure does carry some risk. About 1 in 300 to 500 women will miscarry because of amniocentesis. Your doctor can tell you if it’s important for you to have this.

Chorionic villus sampling (CVS). Doctors take a small piece of your placenta by putting a needle through your belly or a small tube up your vagina. They test the sample for Down syndrome and other genetic conditions. Only some high-risk women will need this test, usually if a screening found a risk of a birth defect. The procedure will tell you for sure if there’s a problem, but it also comes with a risk of miscarriage that’s similar to amniocentesis. Talk to your doctor about whether you should have CVS.

What Do I Do Once I Have the Results?

Prenatal test results can help you make important health care decisions. But it’s important to remember that many of them tell you it’s possible, but not certain, that your baby will be born with a disorder. No test is 100% accurate. 

Talk to your doctor about the results you get and what they mean. A genetics counselor can also help you decide what to do after a positive result and what life will be like for your child if he has a disorder.

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