There are important differences with the two tests. An amnio can be performed at 15 to 20 weeks, and the results can take up to a month to receive. Alternately, a CVS can be done much sooner, at 10 to 12 weeks, and the results are available one to two weeks after the test. The advantage with both tests is that as diagnostic tests, the results are definitive, says Dr. Norton.
Neither test is without risks, most importantly miscarriage, although doctors point as these tests have become more routine over the years, the rate of miscarriage has lessened. Dr. Norton recently published results of incidences of miscarriage using CVS and amniocentesis. The study, which appeared in the September 2006 issue of Obstetrics and Gynecology, found "the risk of both CVS and amnio seems to be lower than the one in 200 risk that is commonly quoted."
"I would suspect that the majority of pregnant women don't really understand these tests [quad serum]," says Dr. Norton. "I think many believe that a 'normal' result means that their fetus is fine, and an 'abnormal' result means it most likely has a problem, when in fact, the test simply refines risk. Most fetuses with an abnormal [positive] result ultimately turn out to be fine, while some fetuses with a normal [negative] result turn out to have Down syndrome, or some other problem that the test doesn't test for."
Why Screening Tests Are Important
While the quad serum and other prenatal screening tests may make your life more stressful, these tests do give you and your healthcare provider important information about how your baby is developing.
Dr. Timothy R.B. Johnson, the chair of the department of Obstetrics and Gynecology at the University of Michigan Health System Hospital in Ann Arbor, Michigan, explains that the quad screen does more than just assess a baby's risk factors for certain disorders. "Abnormal hormone levels are also associated with complications such as preterm labor." So, a healthcare provider might manage your pregnancy differently based on the results.