What’s the Difference Between Screening and Diagnostic Prenatal Tests?

Over the last decade, the field of science has made tremendous progress in understanding the role of genetics in human development and disease. Today, home testing kits make it easy for anyone to gain insight into their own genetic makeup. Interestingly, it was the field of maternal-fetal medicine that helped pave the way for non-invasive screenings to identify risk for genetic disorders. It has been nearly twenty years since prenatal genetic testing aimed at identifying chromosomal abnormalities became a routine part of maternal care.

These days, all of the different prenatal testing options can be overwhelming, particularly for a first-time mother-to-be. It doesn’t help that the terminology relating to genetic testing is sometimes used inconsistently. The difference between the terms “screening test” and “diagnostic test” is a common point of confusion. Although the two words are often used interchangeably, they actually refer to two very different measurements. A screening test measures risk, while a diagnostic test seeks to confirm a medical condition. For a concerned expectant mother, it is essential to clearly understand this distinction.

Understanding Risk and Prenatal Screening

Today, most women are offered non-invasive prenatal testing around the ten-week mark of pregnancy. In medicine a screening is used to identify the possibility, or risk, that a patient has an undiagnosed condition. Non-invasive prenatal testing is a screening since it evaluates only the risk that a fetus has a chromosomal abnormality.

Chromosomes hold human genetic information in the form of DNA. They are passed from parents to children and are vital to ongoing development. Down syndrome is the most well-known chromosomal disorder. While most babies born with Down syndrome can lead long, productive lives with the right care and accommodation, there are others, such as Edwards syndrome, that may lead to a very short lifespan outside of the womb. A sample of maternal blood is all that is needed to give a picture of the risk (or chance) of the fetus having a serious chromosomal issue — but not whether the disease is definitely present.

The vast majority of screenings come back negative, meaning there is very little risk that the fetus has a chromosomal abnormality. Some women do receive results indicating an elevated risk. These expectant mothers are typically offered genetic counseling to understand what this means, as well as subsequent diagnostic testing to determine whether the disorder is in fact present.

Diagnostic Testing Options

The primary diagnostic tests for chromosomal abnormalities are amniocentesis and chorionic villus sampling (CVS). Both use fetal cells for diagnostic evaluation, which provides a more definitive answer to whether the baby will be born with a genetic condition.

Amniocentesis studies amniotic fluid, which contains fetal cells with the baby’s DNA that the fetus sheds as it grows. In this procedure, performed around the 14-week mark, a provider uses a needle to withdraw a fluid sample to be sent for laboratory evaluation. Ultrasound guidance helps the provider safely obtain the sample without harm to the fetus. Results are usually ready in about a week and will confirm the presence of a disorder — or else prove that the screening result was a false positive.

CVS is another diagnostic option. This test uses a small sample of the placenta to assess for fetal issues. CVS can be performed earlier in a pregnancy than amniocentesis, around the 11-week mark.  Like with amniocentesis, these results will also confirm or refute the screening result.

Screening Advantages

Screenings are effective for pregnant women because they are a safe, simple, and relatively low-cost way to understand if a fetus is likely to be born with a genetic condition. These were once only offered to women of advanced maternal age (over 35 years of age) since increasing age is linked to a higher rate of fetal complications. But today, they are offered to all expecting women. Many (but not all) moms-to-be take advantage of the opportunity, and a negative (low risk) result often triggers public declaration of the pregnancy news.

For women who undergo diagnostic testing to confirm or deny a positive screening result, the process is often an anxious one. However, knowing what to expect, can help inform an appropriate birth plan and any subsequent care needs.