Prenatal tests: An overview

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. She'll do a Pap smear (unless you've had one recently) to check for abnormal cells. She may also do a culture to check for chlamydia and gonorrhea.

Next, she'll order routine blood tests to identify your blood type and Rh status, as well as a blood count to check for anemia.

She'll also have the lab test your blood for:

• Syphilis

• Hepatitis B

• Immunity to German measles (rubella)

• Immunity to chicken pox — if you're not sure whether you've ever had the illness or been vaccinated against it.

Finally, she should offer you 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.

Your caregiver will offer you screening tests that can give you information about your baby's risk of having Down syndrome and other chromosomal problems and birth defects.

First-trimester screening options include a blood test that's done between 9 and 13 weeks. If it's available in your area, you'll also be offered a nuchal translucency screening, which is a type of ultrasound that's done at 11 to 13 weeks. Together, the blood test and ultrasound are known as the first-trimester combined screening.

Depending on your ethnic background and medical history, you may want to consider carrier screening to determine whether your baby is at risk for genetic disorders such as cystic fibrosis, sickle cell disease, thalassemia, and Tay-Sachs disease.

Finally, you'll have the option of undergoing chorionic villus sampling (CVS), an invasive genetic diagnostic test that's generally done between 11 and 12 weeks. CVS can tell you whether your baby has Down syndrome or other chromosomal abnormalities. The test will also look for certain genetic disorders that your baby may be at risk for.

NOTE: If you want the option of having CVS after you get the results from your first-trimester screening, you'll need to schedule the screening tests earlier rather than later.

Second-trimester tests

During each of your second-trimester prenatal visits, your practitioner will ask 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 and to verify your baby's due date.

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.

Between 15 and 18 weeks, you may have a multiple marker screening, a blood test that can give you some information about your baby's risk of having certain chromosomal problems and other birth defects. Ideally, this test should be done in conjunction with first-trimester screening tests in what's known as integrated or sequential screening.

Finally, you'll have the option of undergoing amniocentesis, an invasive genetic diagnostic test done between 16 and 20 weeks. Amniocentesis can tell you whether your baby has Down syndrome or other chromosomal abnormalities, neural tube defects, and certain genetic disorders. Some women wait for the results of first-trimester and second-trimester screening tests before deciding whether to have amniocentesis.

Third-trimester tests

During your third-trimester prenatal visits, your practitioner will 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, she'll order 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 she's concerned about your baby's growth, she'll order 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.