New tests offer safer, painless detection of Down syndrome
Los Angeles Times
Such screening typically isn't done until the second trimester, however, and the more reliable tests can, in rare cases, cause a miscarriage.
The options are about to improve. A safe new test can be given as early as the 10th week of pregnancy, researchers have reported, accurately identifying fetuses with Down syndrome.
That test alone is expected to increase the number of women who learn that they are carrying a baby with the genetic disorder.
"That means there is screening available much earlier in pregnancy, which allows more time and more privacy in decision-making," said Dr. David Luthy, director of Swedish Medical Center's Division of Perinatal Medicine.
Another safe second-trimester test also is emerging. When combined, the screenings promise 94 percent accuracy, approaching that of today's more invasive methods.
Details of the new screening methods, reported earlier this month at the Society for Maternal-Fetal Medicine's annual meeting in New Orleans, come from a government-funded study of more than 33,000 pregnant women in the United States.
The study, called the First and Second Trimester Evaluation of Risk (FASTER), calculated the accuracy of using maternal blood tests and sometimes ultrasound to detect Down syndrome. Swedish was one of more than a dozen sites that took part in the trial.
Though the tests, alone or in combination, aren't foolproof, they bring to four the number of noninvasive screenings that indicate whether a woman is carrying a baby with Down syndrome.
The likelihood that a child will have the condition, marked by mental retardation and physical defects, rises with a woman's age. Women of all ages already are offered a second-trimester blood test, called the triple screen, that can detect Down syndrome with about 65 percent accuracy.
Women with positive results are then offered amniocentesis or CVS (chorionic villus sampling) for a definitive diagnosis. In amniocentesis, a needle is used to take a sample of amniotic fluid; in CVS, a sample of placenta is removed. Though highly accurate, both tests pose a slight risk of miscarriage.
That initial screening, however, misses many cases of the disorder and has a high rate of false positives, causing some women unnecessary worry.
In an effort to find more accurate alternatives, researchers in the new study found that a first-trimester maternal blood test, fetal ultrasound and the mother's age can determine with 76 percent accuracy (and a 3.2 percent false-positive rate) whether a fetus has Down syndrome. The ultrasound portion of the exam, called nuchal translucency, measures the thickness of the back of the fetus' neck, which can be an indication of Down syndrome.
The researchers found that a second-trimester blood test measuring the three substances in the current triple-screen regimen — along with a substance called inhibin-A — improves Down syndrome detection to 84 percent, with an 8.4 percent false-positive rate. When a woman undergoes both the first-trimester and second-trimester screenings (called an integrated screening), as all the women in the study did, the detection rate improves to 90 percent, with a false-positive rate of 5.4 percent.
The tests are already offered at some hospitals in the Seattle area, including Swedish, the University of Washington and Eastside Maternal Fetal Medicine.
"Most pregnant women that come to Swedish are choosing the integrated test because it appears to offer the highest detection rate with the lowest false-positive rate," Luthy said.
This has the advantage of reducing the need for having an amniocentesis, but the downside is women don't find out the results until well into the second trimester.
'Up to the individual'
Women can opt for a range of testing or none at all depending on their age, medical history and personal preferences. Women 35 and older, who traditionally are offered amniocentesis or CVS, may choose to skip invasive testing completely if the result of the screening is negative.
"I think it will be up to the individual patient to make that decision," said Dr. Mary E. D'Alton, the principal investigator of the study and chief of obstetrics at Columbia-Presbyterian Hospital Medical Center in New York.
An official recommendation on how doctors should advise women on prenatal screening will be formulated this year along with guidelines on ensuring accurate screening, D'Alton said. Guidelines will be important because nuchal translucency ultrasound requires highly trained sonographers who have experience performing the measurement. It will take time for sonographers nationwide to learn the technique, experts predict.
Seattle Times staff reporter Julia Sommerfeld contributed to this report
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