Michael Urig, MD, is chief of obstetrics and gynecology at Banner Good Samaritan Medical Center in Phoenix. For more information on this topic, talk with your doctor, or call Dr. Urig’s Ahwatukee office at (480) 759-9191
Question: I'm going to have a baby! We are excited and nervous. What do we need to know about prenatal testing?
Answer: Birth defects, ranging from minor to life-threatening, affect three to four percent of all pregnancies. Prenatal blood tests, ultrasound and genetic testing can help identify possible defects. Performing these tests is a decision only an expectant mother can make, taking into account factors such as her health, age, previous pregnancy experience, family history, and moral and ethical beliefs.
Two categories of tests are available, screening and invasive. Screening tests have no risk to the mother or the fetus. One available first trimester test is a combination blood test and specialized ultrasound, which can detect up to 85 percent of Down Syndrome and up to 98 percent of certain chromosomal abnormalities. In the second trimester, a blood test that checks for spinal cord defects, Down Syndrome and Trisomy 18 is offered. Also, an ultrasound performed around 18 weeks can identify 35 to 60 percent of all birth defects, and usually the most significant ones.
Invasive tests include chorionic villous sampling (CVS), which is performed in the first trimester and involves taking a tissue sample from the developing placenta. This test poses a one in 200 miscarriage rate and accurately detects chromosomal abnormalities 99 percent of the time. The test can also check for other genetic diseases based on family history. Another highly accurate test is amniocentesis, conducted in the second trimester, where a sample of the fluid that surrounds the baby is removed and tested. Risk of miscarriage is one in 250.
Invasive testing is generally not recommended unless there is some type of genetic risk or concern. Most importantly, all prenatal testing for birth defects is optional and left to the personal choice of the expectant mother.
Reviewed June 2010