One of the reasons why prenatal visits with an OB or midwife are so important is that they allow you and your healthcare provider to keep track of the baby’s development and health. From measuring the growth of your expanding belly, to ultrasounds and verbal check-ins, your labor and delivery team wants to make sure that all is well.

One of the ways we track your baby’s health is by using routine prenatal tests at different points throughout the pregnancy. While these tests are never 100% accurate, they can give us a better idea of what’s going on inside. The large majority of the time, everything is just fine and you’ll get the all clear.

However, there are times where the results of these tests cause us to investigate a little further. In most cases, this requires the use of diagnostic tests.

prenatal testing

Diagnostic Tests

Diagnostic Tests Are an Important Part of Prenatal Testing

Unlike routine prenatal tests and screening tests, which are fairly numerous, there are only three tests that are considered diagnostic in nature. Diagnostic prenatal tests are typically used in one of three scenarios:

  • You are considered to be “of advanced maternal age, “meaning you are 35-years or older.
  • You received an abnormal result from one or more of the screening tests.
  • You (or the baby’s father) have a personal or family medical history that increases the risk of your baby having a genetic disorder.
  • There is an obvious complication or cause for concern during your pregnancy.

Diagnostic tests are more invasive than their counterparts and carry higher risks to the mother and/or baby, which is why they are not a part of routine testing procedures. Their results can shed more specific light into the baby’s development, and can tell us whether the baby has a genetic disorder, or developmental issue or a blood incompatibility with the mother.

The three diagnostic tests are:

Chorionic Villus Sampling (CVS). Chorionic villus are tiny little outgrowths – like teeny-tiny fingers – that extend from the placenta and help it attach to the uterus. By taking a small sample of chorionic villi tissue, using an ultrasound-guided needle, we get genetic information about the baby’s DNA. The tissue sample will be evaluated by special pathologists, who look for indicators of a chromosomal abnormality.

CVS is typically administered around the ninth to eleventh weeks of pregnancy, which allows us to get results before an amniocentesis could be performed. This is especially helpful because, depending on the results, couples can decide whether or not to proceed with their pregnancy sooner – rather than later. CVS is not a widely available test and is only recommended in very specific circumstances, like abnormal results from first-trimester blood tests or an abnormal nuchal translucency screening.

Amniocentesis. An amniocentesis uses a long needle that is inserted into the uterus to extract amniotic fluid for analysis. It can provide good information about things like genetic disorders, such as down syndrome or cystic fibrosis. Among other things, an amniocentesis will tell us about any blood incompatibilities the baby may have with the mother, certain neural defects or issues with brain and spinal cord development. It can also inform us more about fetal maturity toward the end of pregnancy.

Like CVS, an amniocentesis is usually performed in conjunction with an ultrasound to avoid harm to the baby. It is administered at the 15 to 20-week period to test for chromosomal abnormalities, but things like Rh disease or blood incompatibility can’t be tested until later in the pregnancy. While it is 99.5% safe, amniocentesis is still only performed if a probable risk is determined.

Fetal Maturity Tests. Referenced above, this is a specific amniocentesis, performed in the later stages of the pregnancy to determine whether or not the fetus is mature enough to breathe on its own. If a baby’s lungs are not fully developed, it’s at higher risk for developing respiratory stress syndrome – one of the leading causes of death for newborns.

Fetal maturity tests are often used for women who are two weeks overdue, if a stated due date seems questionable, and/or if complications arise for the mother and/or baby towards the end of pregnancy. Determining whether or not the baby’s lungs are mature will assist your labor and delivery team in making the right decision in regards to labor induction, scheduling a C-section and/or deciding if a neonatal team should be present to care for the baby if an early labor is required.

While none of these prenatal diagnostic tests are mandatory, they can be useful tools for you and your prenatal care provider to learn more about what’s going on for you and your baby.