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May 17, 2007 — Experts addressed in further detail new screening guidelines for fetal chromosomal abnormalities at the 55th Annual Clinical Meeting of The American College of Obstetricians and Gynecologists (ACOG). In January 2007, ACOG revised its guidelines that now recommend offering fetal chromosomal screening to all pregnant women, regardless of age, because of improvements in low-risk, noninvasive screening methods. The experts reviewed what these new recommendations mean, in practical terms, for both women and physicians.
"Although the guidelines have changed to include offering screening to all pregnant women, chromosomal risk assessment and screening have been going on for many years and haven't, in practice, been strictly limited to women age 35 and older," according to James D. Goldberg, M.D., medical director at San Francisco Perinatal Associates in California. He is a member of ACOG's Committee on Practice Bulletins-Obstetrics and immediate past chair of ACOG's Committee on Genetics.
"In the past, many women wanted to be screened for peace of mind or in order to be prepared for a baby with a genetic defect. Essentially, what is different now is that all pregnant women will be informed that screening is available, regardless of whether or not they choose to have it," Dr. Goldberg said. Every year in the United States, approximately 1,900 to 2,000 babies are born with Down syndrome, a genetic disorder caused by an extra chromosome. Down syndrome, also known as trisomy 21, results in mental retardation and, in some babies, serious congenital heart defects. Down syndrome is the most commonly identified cause of mental retardation. A 35-year-old woman has a one in 92 chance of having a baby with any chromosomal disorder compared with a one in 476 chance for a 25-year-old woman. The risk of having a baby with Down syndrome is one in 378 for a 35-year-old woman and one in 1,250 for a 25-year-old. "Although older women are at higher risk of having a baby with a chromosomal defect, the majority of babies with Down syndrome are born to women younger than 35 because well over 80 percent of all babies in the United States are born to these women," Dr. Goldberg said.
"There has been some public misunderstanding about the change in ACOG's guidelines," said Deborah Driscoll, M.D., professor and chair of the department of ob-gyn at the University of Pennsylvania in Philadelphia. Dr. Driscoll was lead author of ACOG's Practice Bulletin "Screening for Fetal Chromosomal Abnormalities" and is vice chair of ACOG's Committee on Practice Bulletins-Obstetrics, which developed the guidelines with ACOG's Committee on Genetics and the Society for Maternal-Fetal Medicine.
"We are not recommending that all pregnant women be screened, but rather we are recommending that all pregnant women be offered screening. Physicians are ethically obligated to fully inform our patients of their health care options, including prenatal testing. It is entirely up to the patient to decide whether or not she wishes to be screened for fetal chromosomal abnormalities without judgment from the physician."
According to ACOG's new guidelines, screening for Down syndrome should occur before the 20th week of pregnancy. ACOG advises that all pregnant women, regardless of their age, also should have the option of diagnostic testing. A woman's decision to have an amniocentesis or chorionic villus sampling (CVS) is based on many factors, such as a family or personal history of birth defects, the risk that the fetus will have a chromosomal abnormality or an inherited condition, the consequences of having an affected child, the risk of pregnancy loss from an invasive procedure, and the options for earlier termination.
"The goal is to offer screening tests with high detection rates and low false-positive rates and also to offer patients diagnostic testing options if the screening test indicates an increased risk for having a child with a chromosomal abnormality," said Dr. Driscoll. Screening is also available for other chromosomal abnormalities such as trisomy 18, trisomy 13 and Turner's syndrome, as well as for neural tube defects. The ACOG guidelines discuss the advantages and disadvantages of each screening test and some of the factors that determine which test should be offered, including gestational age at first prenatal visit, number of fetuses, previous obstetrical and family history, and availability of various screening tests.
"One of the difficult issues surrounding screening tests for Down syndrome and other genetic disorders is that they do not give simple 'positive' or 'negative' results," according to Dr. Goldberg. "What screening test results provide is a risk number. This number indicates the level of risk that the fetus has a genetic problem, based on the woman's age and the test results."
The main disadvantage of screening is the potential for false-positives or false-negatives, meaning some fetuses with abnormalities will not be detected, while other, healthy fetuses may wrongly be flagged for Down syndrome. However, invasive diagnostic testing detects 100 percent of autosomal trisomies, which includes Down syndrome, trisomy 18 and trisomy 13.
"Genetic counseling by professionals is very important for our patients," added Dr. Driscoll. "Ob-gyns need to be well-informed about Down syndrome so that they can provide their patients with accurate information and relevant resource referrals so that they can make informed decisions."