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Dr. Jenny Pennycook, St. Luke's Hospital

Prenatal testing can identify specific genetic conditions

Prenatal medicine has come a long way in the diagnosis of fetal conditions during pregnancy. In the past, detecting many conditions involved invasive testing with amniocentesis, or they were not diagnosed until after birth. Advances in ultrasound technology, cell biology and genetics now offer new options for testing.

In general, testing can be either screening or diagnostic. Screening tests are used to detect or predict the presence of a disease or other abnormality. Diagnostic tests are used for confirmation in a patient who is already suspected of having a disease or other abnormality.

Previously, screening blood tests only included the quad screen and the sequential or integrated screen which offer final results in the second trimester.

The newest test available is non-invasive prenatal testing (NIPT) with cell-free fetal DNA. This test can be performed as early as ten weeks gestation with a simple blood draw for women determined to be high-risk for specific fetal genetic conditions (such as Down syndrome, Trisomy 18 and Trisomy 13) with no risk to the fetus. This can provide reassurance if the findings indicate the baby is at low risk for the conditions, or it can provide early diagnosis of a probable genetic condition.

Patients who would benefit from this test include women over 35, women with a prior pregnancy affected by these conditions, women with ultrasound findings which indicate a potential condition or women with other abnormal test results or risk factors as determined by a discussion with one's doctor. The test detects "bits" of DNA from the baby that are in the maternal blood circulation. A major advantage of this test is a high detection rate; about 98 to 99 percent of cases of Down syndrome are detected by this test. More importantly, it has a low false-positive rate. What this means for the patient is more dependable results than previous screening options.

While the test is extremely accurate, it does have some limitations. It does not replace the precision of a diagnostic test such as amniocentesis, and it only tests for specific genetic conditions. Also, in a very small number of cases, a result cannot be obtained. It is important to discuss these limitations and all of your screening options with your doctor so that you can make an informed choice.

Dr. Jenny Pennycook at St. Luke's Hospital is trained in all aspects of general obstetrics and gynecology and enjoys helping patients manage their health through preventative wellness. She specializes in high-risk pregnancies, contraception options, laparoscopic surgery and adolescent care. Call 636-936-8777 or visit stlukes-stl.com.

This article was published in the St. Louis Post-Dispatch on January 23, 2014.