Prenatal testing

What is prenatal testing?

Prenatal tests – including prenatal blood tests, urine tests, and ultrasounds – are one of the many ways your healthcare practitioner will check on the well-being of you and your growing baby.

In most cases, prenatal tests provide reassurance that everything is just fine. In some cases, they alert you and your caregiver to a condition that can be remedied with proper care (such as iron supplements for anemia). And, in other cases, tests can help determine that there's a pregnancy complication that needs careful monitoring or that your baby may have a genetic condition.

You'll have screening tests and diagnostic tests. Screening tests assess the risk of something. For example, a screening test may tell you that you're at risk for gestational diabetes or that there's a chance your baby may have a birth defect. A screening test poses no risk to you or your baby. It isn't definitive, though – meaning it can't give you a result that's 100-percent certain.

A diagnostic test, on the other hand, can confirm a condition or defect. It often follows a screening test, though it can also be done instead of screening. Diagnostic tests such as amniocentesis and chorionic villus sampling carry a small risk to the pregnancy.

Some of the prenatal tests you'll receive are routinely offered during pregnancy, while others are optional or done to follow up on or address a potential concern. Some are done several times during pregnancy, while others are done only once, if necessary.

In general, any genetic testing is optional. It's the job of your doctor or midwife to explain your options and the pros and cons of each so you can figure out what's right for you.

Here are the prenatal tests you'll be offered during your pregnancy. Some will happen at your regular prenatal visits, and some will be scheduled at other times. Note that some tests need to be done during certain weeks of pregnancy, so if you're going to be traveling, make sure there isn't a test you'll miss during that time. Usually you'll have a window of a few weeks to work with.

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Your prenatal testing options at a glance

First-trimester prenatal testing

There's a lot to do right off the bat! Here 's what will come up in your first trimester:

Pelvic exam and cultures

At your first prenatal visit, your practitioner will give you a thorough physical, including a breast and pelvic exam. If your Pap smear isn't up to date, your practitioner will perform one to check for cancerous or precancerous cells on your cervix. You may also have a culture to check for chlamydia and gonorrhea.

Ultrasound

Some women will have an ultrasound in their doctor's office during their first prenatal visit. This ultrasound is useful to accurately confirm your due date, confirm the pregnancy is in your uterus (and isn't ectopic) and to determine whether you're pregnant with one baby or twins or multiples.

Your practitioner may especially recommend an early ultrasound if you've had a pregnancy complication in the past, such as a miscarriage or ectopic pregnancy.

Blood tests

Your practitioner will order a panel of blood tests standard for every pregnant woman. This includes:

• Blood type
• Rh factor
• Anemia
• Syphilis
• Hepatitis B and hepatitis C
• HIV
• Rubella immunity
• Chicken pox immunity (some providers will check)
• Toxoplasmosis
• Thyroid test (some providers will check)

Urine sample

Your practitioner will ask for a urine sample at your initial visit to test for:

• Asymptomatic urinary tract infections. Pregnant women are more likely to get kidney infections than nonpregnant women.

At every visit, your practitioner may ask for a urine sample to check quickly for:

• Glucose (high levels of blood sugar are a sign of diabetes)
• Protein. High protein levels may be a sign of preeclampsia, a serious pregnancy complication.

Glucose challenge test

If you're at high risk for gestational diabetes, a glucose challenge test might be done at your first visit. Otherwise it will be done at the end of your second trimester.

TB test

In some cases, your practitioner will also do a skin test to see if you've been exposed to tuberculosis.

Screening tests for genetic or other conditions

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:

• First trimester combined screening. This includes a blood test that's done at 11 to 13 weeks and a nuchal translucency screening, which is a type of ultrasound that's also done at 11 to 13 weeks.
• Noninvasive prenatal testing (NIPT). This is another blood test, also called cell-free fetal DNA screening. It can detect Down syndrome and a few other conditions at 10 weeks of pregnancy or later. It can also tell you the sex of the baby.
• Carrier screening. If you haven't had it already, your provider should recommend expanded carrier screening to determine whether your baby is at risk for any of about 100 genetic disorders such as cystic fibrosis, fragile x, spinal muscular atrophy (SMA), sickle cell disease, thalassemia, and Tay-Sachs disease. As this is a test of your DNA, which doesn't change, you only need to test once in your life.

Chorionic villus sampling (CVS)

If your baby is at increased risk for genetic or chromosomal problems (or if the results of screening tests indicate a need for more testing), you may choose to have chorionic villus sampling (CVS).

This is an invasive diagnostic test that's generally done at about 10 to 13 weeks. CVS can tell you for sure whether your baby has Down syndrome or almost any other chromosomal abnormalities. The test can also diagnose other genetic disorders.

NOTE: If you want the option of having CVS after you get the results from first-trimester screening or NIPT, you'll need to schedule the screening tests as early as possible.

Second-trimester prenatal testing

Urine tests

During each of your second-trimester prenatal visits, your practitioner may continue to ask you for a urine sample to screen for signs of preeclampsia, urinary tract infections, and other conditions.

Alpha Fetal Protein (AFP)

Around 16 to 18 weeks, you'll be offered a test for Alpha Fetal Protein (AFP) to help screen for neural tube defects (problems with the brain and spinal cord), such as spina bifida. If you screen positive, your doctor will refer you to a specialist for a comprehensive ultrasound.

If at the time of this test you haven't had a first trimester screen or NIPT but want screening for Down syndrome, your practitioner can order a quad screen. This includes the AFP screening. The quad screen isn't nearly as accurate as the other tests for chromosomal abnormalities, like Down syndrome, but it's occasionally used.

Ultrasound

Most practitioners routinely order an ultrasound between 18 and 22 weeks. This ultrasound – also called an anatomy ultrasound – checks for physical abnormalities. It also measures your baby's size, checks the location of your placenta, the amount of amniotic fluid, and your cervical length. If you don't already know your baby's sex, you can find out, if you like, at this mid-pregnancy ultrasound.

Glucose screening

Between 24 and 28 weeks, you'll be given a glucose screening test to check for gestational diabetes. This test involves drinking a sweet liquid, waiting an hour, and having a blood test to measure your glucose levels. If your levels are high, your practitioner will follow up with a glucose tolerance test. For that test, you'll need to fast for 8 hours and have blood tests before and after drinking the sweet liquid.

Blood tests

In addition to screening for diabetes and neural tube defects, your practitioner may give you blood tests to check for:

• Anemia. You were probably checked for anemia in your first trimester, but it's common to develop anemia as your pregnancy progresses, even if you're not anemic at first.
• Rh antibodies. If you're Rh-negative, an extra tube of blood may be drawn to check for Rh antibodies before you're given an injection of Rh immune globulin (Rhogam) at 28 weeks.
• Second tests for HIV and/or syphilis

Amniocentesis

You'll have the option of undergoing amniocentesis, an invasive diagnostic test done after 15 weeks. Amniocentesis can tell you for sure 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 prenatal testing

Urine tests

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

Group B strep test

Between 36 and 37 weeks, you'll be tested for a bacteria called group B strep (GBS). We know that babies of women who have this very common bacteria are at higher risk of infection after delivery. So if your test is positive, you'll be given antibiotics to reduce the risk of passing the bacteria 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.)

Blood tests

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.

Ultrasound

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 (such as your baby's growth), you may have additional ultrasounds to make sure your baby's thriving and to check on your amniotic fluid level. Your practitioner may also assess your baby's heart rate with a nonstress test. A combination of the nonstress test and fetal ultrasound is called a biophysical profile (BPP).

Prenatal testing if you go past your due date

If your pregnancy is normal but you go past your due date, you'll probably have testing to make sure your baby's still doing well. This will likely include an ultrasound and a non-stress test (NST).

After your due date, your practitioner uses these tests to make sure it's safe for your baby to continue waiting for labor to start on its own.

Due Date Calculator

Find out your due date, and see how big your baby is right now.

If you're feeling overwhelmed by the number and types of tests, you're not alone. Be sure to discuss any questions you have about prenatal testing with your doctor or midwife, including what's necessary or optional, why a test is being done, and what the results mean.

Learn more:

• Midwife or doctor: Which is right for you?
• How to choose an ob-gyn