Today, health care professionals such as obstetricians, primary care providers, genetic counselors and medical geneticists can help interpret what prenatal screening and tests can and cannot tell. Importantly, diagnostic testing provides greater certainty compared to screening tests.

Early-stage screening

Genetic screening can occur pre-pregnancy or during pregnancy, even at the embryo stage. Carrier screening for genetic conditions such as cystic fibrosis have become part of routine prenatal care. Patients determined to be a carrier of a genetic condition have no signs or symptoms, but their children are at greater risk. Patients can now have screening tests to check to see if they are a carrier for hundreds of genetic conditions. And patients have many options when screening and testing for abnormal chromosome number in a fetus.

Diagnostic testing provides greater certainty compared to screening tests.

NIPS (non-invasive prenatal screening) is the newest and most sensitive approach to Down syndrome screening. It relies on fetal DNA fragments in the mother’s blood that are shed from the placenta. Conditions other than Down syndrome can be screened, but compared to amniocentesis and CVS the amount of information provided is much less. It is important to note that when using NIPS there can be false positives (i.e. a positive screening result that does NOT mean a fetus is affected). NIPS offers the lowest false positive rate when compared to other screening methods. All positive screening test results should be confirmed. Prenatal confirmation requires amniocentesis or CVS.

A range of offerings

If a patient desires the most information, diagnostic testing via amniocentesis or chorionic villus sampling (CVS) should be considered. Amniocentesis is generally performed at 16-20 weeks of pregnancy and CVS at 11-13 weeks. For each procedure, a sample is sent to the laboratory for chromosome analysis. An additional test called a microarray (where smaller parts of chromosomes/DNA segments are analyzed) can also be performed where one can test for hundreds of other genetic disorders.

It is always the patient’s decision whether or not to opt for diagnostic testing, screening or no screening, or testing at all.  

The genetic screening and diagnostic testing options available to patients can make prenatal care visits complex. Health care providers can use guidelines and educational materials from national organizations like the American College of Medical Genetics and Genomics to help explain these concepts. Medical genetics professionals, such as genetic counselors and board-certified medical geneticists, are available to help interpret what the tests can and cannot tell.