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The American College of Obstetricians and Gynecologists (ACOG) released new practice guidelines in 2007 recommending all pregnant women, regardless of their age, be offered screening for Down syndrome.
“A new screening method that combines blood tests with an ultrasound exam is recommended.”
“A new screening method that combines blood tests with an ultrasound exam is recommended.”
The goals of the new guidelines are to offer screening tests with high detection rates and low false-positive rates. In the past, women over the age of 35 were offered amniocentesis or chorionic villus sampling (CVS), both of which are invasive diagnostic procedures. Breakthroughs in first-trimester screening in the past few years have led to the development of several alternative screening options.
The guidelines outline several screening options available for physicians to offer patients. It is not practical or possible for physicians to offer every screening option. The guidelines recommend that physicians choose which strategies to offer based on which tests are available in the area and what best meets the needs of the patient base in the area. Information about the detection and false-positive rates, advantages, disadvantages, alternatives, risks, benefits and limitations should be available to patients for all screening options available in the area.
The newest method of screening is highly recommended by ACOG: a screening conducted during the first trimester that combines blood tests with an ultrasound exam called a nuchal translucency test. The nuchal translucency test measures accumulated fluid behind the fetal neck. When combined with blood tests, this method results in higher detection rates than a second-trimester screen and similar false-positive rates. The guidelines recommend pregnant women of all ages consider screening options before their 20th week of pregnancy.
Approximately one in 800 babies is born with Down syndrome (also known as trisomy 21), a genetic condition where having an extra chromosome causes some mental retardation and often congenital heart defects. Down syndrome accounts for at least half of all babies born with chromosomal abnormalities. Most Down syndrome is caused by an error in cell division called nondisjunction. This error occurs during conception but researchers do not know why it occurs. Researchers do know that the incidence of Down syndrome increases with the advancing age of the mother. It is a gradual increase, the chance of having a baby with Down syndrome goes from about one in 1,500 at age 25 to about one in 300 at age 35. As a result of the gradual increase and the less invasive screening options available, the guidelines have omitted the age 35 trigger for screening.
Historically, women who gave birth at age 35 or older were identified as having the highest risk of having a baby with Down syndrome. These women were offered genetic counseling and an amniocentesis or CVS to screen for Down syndrome.
Amniocentesis and CVS testing were at one time the only methods to check for chromosomal abnormalities. These diagnostic tests have always been reserved for women in a higher risk age group (over 35), women with a history, and women with an abnormal screen because both procedures carry a slight increased risk of miscarriage.
An amniocentesis for the purpose of detecting Down syndrome is typically conducted between 15 and 20 weeks. It is performed under continuous ultrasound guidance. A doctor identifies a pocket of amniotic fluid a safe distance from the baby. A needle is then inserted through the abdominal wall and 15-20cc (3-4 teaspoons) of amniotic fluid is withdrawn. The amniotic fluid contains enzymes, proteins, hormones and cells shed by the fetus. Those cells contain genetic information that is used to diagnose chromosomal abnormalities and other conditions.
Similarly, CVS may be used for genetic and chromosome testing in the first trimester of pregnancy. Using ultrasound as a guide, the doctor inserts a thin catheter through the vagina and cervix or a needle through the abdomen to take a tiny tissue sample from the placenta. The cells are then cultured allowing an accurate determination of any chromosomal abnormalities.
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With these new guidelines, the American College of Obstetricians and Gynecologists (ACOG) is enhancing a woman’s ability to make the choice if she wants to have tests performed to give more information on the potential for Down syndrome. Advances in the technology of screening processes have allowed physicians to take a woman from an age-based population risk of having a baby with a chromosomal abnormality to an individual risk. As a result of the less-invasive screening options, any pregnant woman can have one or more of several screening tests.
“Advances in the technology of screening processes have allowed physicians to take a woman from an age-based population risk ... to an individual risk.”
In the U.S., amniocentesis became a common procedure in the early 1970s for women over the age of 35. CVS followed and became common in the 1980s. Physicians chose age 35 to talk to women about amniocentesis or CVS because they believed age 35 was where the risk of losing a baby from amniocentesis was thought to be fairly equal to the risk of a woman having a baby with Down syndrome or another chromosomal abnormality.
In 1984, the very first screening test became available. They were only looking at a single marker, which contributed to not very accurate testing. Over the next 20 years, we developed a first-trimester test (combined blood test and nuchal translucency test) and a second-trimester test that looks at four markers. In 1984 the detection rate was around 20 percent; currently the detection rate is 70-80 percent.
Testing procedures for chromosomal abnormality have really advanced over the past few years. We know that early in pregnancy (10-14 weeks) there can be a little area of fluid collection behind the neck of the fetus. If that fluid collection is greater than 3 millimeters it is abnormal under any circumstances. It may have several causes, including chromosomal abnormality, a congenital heart defect or a genetic syndrome.
“A woman in a higher risk percentage of having a baby with Down syndrome ... may choose ... a larger hospital setting where extra medical attention can be given.”
At the University of Kentucky this type of screening is offered in the first trimester; the area behind the neck of the fetus is measured in a nuchal translucency test (NT) and is combined with a blood draw to screen for chromosomal abnormalities. The blood is analyzed for two specific pregnancy hormones, PAPP-A and β-hCG. The combination of these tests helps us calculate an individual risk.
Second-trimester screening is also available at UK, called a quadruple marker screen. The quadruple screen detects approximately 79 percent of cases of Down syndrome and is performed between 15 and 21 weeks of gestation. It is a series of four blood tests that measure the amounts of alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), estriol (uE3), and inhibin-A in your blood. When reviewed together, this set of blood tests give information on possible abnormalities in the fetus.
If a patient has an abnormal quadruple screen test or an abnormal first-trimester test, genetic counseling will be offered and an amniocentesis or a CVS test can be considered.
There are several things to think about before deciding to have your baby screened for chromosomal abnormalities. First, try to think beyond the “here and now” and think about what you will do with the information the test shows. Second, determine whether you will be satisfied with knowing if you are high or low risk. Will you want to proceed to a diagnostic test and incur a small risk of pregnancy loss if you are determined to be at a higher risk?
Even if a woman would never consider termination, some women may want to have screening tests in order to prepare. Additionally, babies born with Down syndrome often need extra medical attention at birth. If a screening test places a woman in a higher risk percentage of having a baby with Down syndrome, she may choose to have the baby in a larger hospital setting where extra medical attention can be given. Conversely, there are clearly some women who would prefer to not have a screening test at all. I often hear women say “I get what I get.” Each woman is different and all choices should be respected.
It is important to think about what is best for you and your family and to talk with your doctors about all of your options.
Dr. Hansen is an OB/GYN at UK Chandler Hospital, as well as an associate professor of obstetrics and gynecology at UK College of Medicine.
Each issue of Advances & Insights summarizes an important piece of medical news, accompanied by commentary from a UK expert.
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