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Prenatal Testing

During pregnancy there are specific tests that can be done to ensure that a woman and her baby are in good health. Below are some of the various tests that can be done throughout pregnancy, and some that are done during specific trimesters. These tests can provide useful information about your baby to help prepare you for the day when it arrives. We will go over the tests themselves and discuss reasons why some people choose to have certain tests done, and why some choose not to have them done at all.

General Prenatal Tests

There are some tests in particular that you should definitely have done in the early stages of your pregnancy. Blood tests are a given, and in fact many blood tests are legally required as part of prenatal care. Other tests must be legally offered to the expectant parent as an option. Sometimes parents get these issues confused. For example, all pregnant women have to be tested for blood count, blood type, Rh factor, antibody screen, immunity to Rubella, HIV, syphilis, gonorrhea, and chlamydia. In contrast, all pregnant women must be offered screening tests for birth defects, but there is no requirement to do them.

Urine testing is done quite often during pregnancy. Early on urine is checked for protein and sugar levels, and for infection. During each subsequent prenatal visit (PNV), urine is again checked for protein and sugar.

The last of the basic tests is the ultrasound. This test should be done a few times throughout the pregnancy because there are many things you can learn about your baby as time passes. The abdomen and pelvis are scanned to create a sonogram of the baby and placenta. There are many different types of ultrasound exams that can be performed depending on the area of interest; these include abdominal, vaginal, pulsed doppler, fetal echocardiography, and 3-D ultrasound.

First Trimester Prenatal Tests

In the first trimester, normally the legally required tests are done early in pregnancy, maybe even at your first prenatal visit. There are also optional genetic tests that can be done.

One of the optional genetic tests is called the Nuchal Translucency (NT) Screening test. Normally done at 12 weeks of pregnancy, this test combines ultrasound and a blood test to help determine the risk for Down Syndrome (Trisomy 21) and Trisomy 18. This is a non-invasive screening test, and there is no specific risk involved other than that of a false-positive or false-negative result. Positive screening tests warrant additional testing.

Another optional genetic test is called Chorionic Villus Sampling (or CVS). This type of diagnostic test is referred to as an “invasive test.” A CVS is a medical procedure done by a highly skilled physician who uses ultrasound guidance to insert a narrow, flexible catheter into the placenta in order to obtain cells for fetal DNA analysis. CVS can be done at 10 weeks or later, and usually not past 14 weeks. This test might be done in cases of advanced maternal age—when a pregnant woman is over age 35—but realistically it is not normally done until the patient is closer to age 39 or 40, due to the risks of pregnancy loss. CVS might also be done at any age if the couple has previously had a child or an earlier pregnancy with a DNA abnormality, or it might be done if an early ultrasound (or perhaps the NT test mentioned above) comes back as abnormal.

A less invasive alternative to CVS, NIPT testing, is done during the first trimester. NIPT stands for non-invasive prenatal testing (non-invasive means no risk to the fetus). Like CVS, NIPT looks for chromosomal disorders such as Down syndrome and others. Unlike CVS, NIPT requires only a blood sample from the mother, rather than a uterine tissue sample.

The NIPT test is relatively new, but is fast becoming one of the greatest advances we have in modern obstetrical prenatal genetic testing. Every pregnant woman should seriously consider having one of these tests done, and they can be done as early as 10 weeks of pregnancy. You find out the sex of your baby, and you also learn about possible chromosomal genetic disorders, with results available in one to two weeks.

Second Trimester Prenatal Tests

The Quad Screen blood test (also called the afp test—afp stands for alphafetoprotein) measures four substances in the blood; AFP, hCG (this is the pregnancy hormone), Estriol, and Inhibin-A. This test can help identify the risk of Down Syndrome, Trisomy 18, and spina bifida, and is normally done at 16 weeks of pregnancy. If NIPT or CVS have been done, then this test is modified to look just for spina bifida, which is a birth defect involving the brain and/or spinal cord.

A routine ultrasound for fetal anatomy is often done about 18–20 weeks. Some Ob offices provide this; some refer the patient to an outside specialist. The ultrasound exam evaluates the head-to-toe fetal anatomy, placenta, cord, amniotic fluid, and perhaps the cervix as well, looking for any abnormality that might require follow-up. Our group calls this your “Fifty-point checklist,” like when you take your car in for routine servicing!

Amniocentesis is a genetic test done about 17–18 weeks. This is another “invasive prenatal test” done by a specialist. A needle is inserted into the amniotic sac, a small amount of amniotic fluid is withdrawn, and then the DNA of the fetal cells that float in the fluid is analyzed. The results are the same as with a CVS, but most experts feel that the risk of pregnancy loss with this test is lower than with CVS.

There are many indications for amniocentesis. These include: pregnancy in a woman 35 or older, a family history of a genetic disease or chromosomal disorder, an abnormal result from either the NT test or the Quad Screen test, or an abnormal finding seen during a prenatal ultrasound.

A PUBS, or Percutaneous Umbilical Blood Sampling, is a diagnostic test that examines the blood of the fetus to detect abnormalities. The procedure is similar to amniocentesis, except that the goal is to retrieve fetal blood, not amniotic fluid. This is a highly specialized test which is rarely done except for specific clinical conditions.

Third Trimester Prenatal Tests

There are a few routine tests normally done in the third trimester. One is the glucose screening test to check for possible gestational diabetes, usually done with a blood test for anemia as well. This is also when an Rh-negative mother is supposed to get her injection of Rh-immunoglobin, often referred to as Rho-Gam.

Another routine test is the Group B strep vaginal screening test usually done about 35-36 weeks.

Other third trimester tests that are not routine but are common are ultrasound, fetal non-stress testing (NST) and biophysical profiles (BPP). Many ob providers perform at least one routine ultrasound in the third trimester but for a normal low-risk pregnancy this test is not yet “required” (although in my opinion it should be). An ultrasound at this stage can confirm that the baby is head down and can check for fetal growth and amniotic fluid volume.

A fetal NST is normally done after about 32 weeks, and typically once NSTs are started, they continue for the rest of the pregnancy—sometimes weekly, sometimes twice a week—until delivery.

There are many indications for NST: decreased fetal movement, babies that measure too small or too big on ultrasound, decreased amniotic fluid volume, maternal high blood pressure or gestational diabetes, overdue pregnancy, and high-risk pregnancy based on other conditions.

A BioPhysical Profile test combines an ultrasound with an NST. BPP examines fetal breathing, fetal movement and muscle tone, fetal heart rate, and amniotic fluid volume. This test is usually done when the baby’s health is in question and perhaps the baby did not pass the NST.

Optional Genetic Tests

It is important to understand that there is no requirement to have the above genetic tests done; they are optional. The ob care provider is required to explain these tests and offer them to the pregnant couple. In turn, the couple must make the final decision regarding which test or tests to do, or they may choose to decline genetic testing altogether.

Many of the conditions tested for with prenatal genetic testing are incurable. One reason to test for these conditions is that when one of them is identified, the couple can choose to end the pregnancy. Another reason is to help prepare for the birth of a special needs child. Lastly, certain conditions might convert the pregnancy to high-risk, which can require additional fetal monitoring, involvement of specialists, delivery in a high-level medical center or possibly a planned Cesarean birth.

Couples decline genetic testing for different reasons. Some believe strongly that no matter what condition or disorder their baby has, it will not affect their decision to continue the pregnancy. Therefore, they do not want to risk losing the pregnancy if there is a complication from having an invasive procedure. Sometimes this is referred to as “letting nature take its course.”

Non-invasive screening and testing does not pose a risk of pregnancy loss, but there is a risk of a false-positive or a false-negative result. A false positive result means that the test came back positive, showing an abnormality or a possible abnormality, but then with further testing (sometimes done during the pregnancy and sometimes not done until after the delivery) it may be determined that the condition was not present at all. The frustrating aspect of a false positive is that it takes additional testing to find out if indeed the test result was wrong. When a fire alarm goes off, you do not know until you investigate whether there is really a fire or it is just a false alarm.

A false negative test is the opposite of a false positive, and is potentially even scarier. This is when a test that is supposed to help identify a potential problem or abnormality comes back as normal, despite the fact that the fetus actually does have something wrong. Most prenatal tests are designed so that the chance of a false-negative test is very low. This is one reason why there are so many false positives. The tests are designed to be sensitive so that an initial positive test can be followed up to quickly determine if the test is a false-positive or a true-positive. The worst test we have in the world of prenatal testing is a false-negative. This is when the doctor and the patient think everything is normal, and then find out later in pregnancy (or sometimes even after the baby is born) that a serious condition is present that was missed by the test. It does not help to have a fire alarm if it fails to go off when there actually is a fire.

    [Page updated June 2017]