Wednesday, January 19, 2011
Posted by: Wendy Busch, MS, CGC
There is some misinformation and confusion surrounding prenatal genetic screening. Prenatal genetic screening is the process of refining a woman’s probability or risk for having a child with the chromosomal abnormalities Down syndrome and trisomy 18. Every pregnancy carries with it some risk of these disorders. And we know that this risk increases with the age of the mother. Screening results in what we call “revised probability” for the individual patient. It doesn’t provide a “yes” or “no” answer, nor does it test for every possible abnormality or disease. Instead, it provides a pregnant woman with a better understanding of the chance that her baby could have one of these disorders.
Prenatal Genetic Screening - A Personal Decision
We offer all of our pregnant patients the option of prenatal genetic screening, but the decision is truly a personal one. When I talk with women and their partners about screening, I work with them to determine how anxious or concerned they are about the pregnancy to begin with. If they’re not at all concerned, genetic screening by itself can sometimes cause some anxiety. If the family has some concern or is interested in more information, then we discuss whether they’re comfortable with the concept of probability or if they need definitive answers. If they need definitive answers, some may skip the screening step and go straight to diagnostic testing, such as amniocentesis or chorionic villus sampling (CVS). If the family has concerns about the pregnancy but isn’t sure whether they would undergo invasive testing, genetic screening might be helpful for them in making that decision.
Prenatal Genetic Screening Today
Recently we’ve transitioned from what was known as “First Trimester Screening” to a protocol known as the “Sequential Screen” because it offers better detection rates. Part 1 of the Sequential Screen is completed during the first trimester. It includes a blood test and a nuchal translucency (NT) measurement. The blood test measures beta human chorionic gonadotropin (B-hCG) and pregnancy-associated plasma protein-A (PAPPA-A). Certain levels of these proteins are associated with Down syndrome and trisomy 18. The NT measurement is obtained by ultrasound and focuses an area on the fetal neck that can be thicker when a chromosome disorder is present.
Results from the blood test, the NT measurement, and the mother’s age-related risk are entered into a computer program. The computer-generated result is expressed as a probability. For example, if the result is “1/1,000” this means there is a one in 1,000 or .001% chance that the baby will be born with Down syndrome or trisomy 18.
A result of 1/50 means a woman has a 2% chance of having a child with one of these chromosome disorders. For Part 1 of the Sequential Screen, we consider this “positive.” Positive in this case doesn’t mean the baby will be affected, just that there is an increased risk. For women who would like more information at this point, we offer amniocentesis or CVS. These tests can diagnose not only the chromosome disorders addressed in the screening process, but others, as well. They are both invasive tests that carry some risk of miscarriage; nationally the risk of pregnancy loss is around one in 200.
If the result from Part 1 of the Sequential Screen indicates a risk lower than 1 in 50, we suggest that the mother complete Part 2, which is a blood test in the second trimester. The Part 2 blood test screens for four additional markers* which can be associated with Down syndrome and trisomy 18, as well as open neural tube defects. At this step, we consider a risk of 1/270 (.37%) or greater to be “positive” and offer diagnostic testing. During the second trimester (up to 22 weeks), diagnostic testing is completed using amniocentesis.
Pros and Cons of Prenatal Genetic Screening
The advantage of prenatal genetic screening is that it can serve to reassure families and reduce the number of invasive diagnostic tests performed. The advantage of Sequential Screening, specifically, is that families have an earlier risk assessment and earlier access to diagnostic testing, if they wish. The downside is that screening doesn’t check for everything and never provides a “yes” or “no” answer.
There is a misconception that genetic screening and diagnostic testing are only for families who would consider terminating a pregnancy. Many families want definitive answers so they can contact local support networks, plan for the proper pediatric care for their newborn and prepare their families. The physical and emotional intensity of having a normal baby can be overwhelming and having to deal with an unexpected diagnosis at birth only increases that.
Healthcare offers many, many tests. The bottom line is a test is only good if it is helpful to you. I remind the families I meet with that the majority of results obtained from genetic screening point to a healthy outcome; most women who have positive screening results will go on to have healthy babies.
Wendy Busch, MS, CGC, is one of three Certified Genetic Counselors who work with women and their families at Northwest Perinatal Center.
*Part 2 of the Sequential Screen tests for maternal serum alpha-fetoprotein (MSAFP), hCG, estriol and inhibin A.