MEDICATIONS: POSITION ON FIRST TRIMESTER SCREENING METHODS

June 30, 2004 — First-trimester screening for genetic defects is now an option for pregnant women, but only if certain criteria are met, according to a new Committee Opinion issued today by The American College of Obstetricians and Gynecologists (ACOG). New technologies, such as measuring nuchal translucency (NT), have allowed for earlier, noninvasive screening for chromosomal abnormalities and, when combined with serum screening in the first trimester, have comparable detection rates as standard second-trimester screening.

First-trimester screening offers several potential advantages over second-trimester screening. When test results are negative, it may help reduce maternal anxiety earlier. If results are positive, it allows women to take advantage of first-trimester prenatal diagnosis by chorionic villus sampling (CVS) at 10 to12 weeks or second-trimester amniocentesis (> 15 weeks). Detecting problems earlier in the pregnancy may allow women to prepare for a child with health problems. It also affords women greater privacy and less health risk if they elect to terminate the pregnancy.

In the past decade, research has shown an association between fetuses with certain chromosomal abnormalities and ultrasonographic findings of an abnormally increased NT (an area at the back of the fetal neck) between 10 and 14 weeks gestation. The newer first-trimester screening method includes measurement of NT, free beta subunit of human chorionic gonadotropin (*-hCG), and pregnancy-associated plasma protein-A (PAPP-A). It has a comparable detection rate for Down syndrome as the more commonly used second-trimester screening using four serum markers (alpha-fetoprotein, *-hCG, unconjugated estriol, inhibin-A). Women who screen positive are at an increased risk for having a child with Down syndrome. These women may then decide to have a diagnostic test such as amniocentesis or CVS to determine if the fetus is affected since screening tests can give false positive results.

"Researchers developed this new method of testing because women want to know earlier in their pregnancies if there are any problems. Many women also want to avoid having an invasive diagnostic procedure such as CVS, which carries a small risk of miscarriage. However, it is important for women to recognize that a negative screen indicates that their risk of having a child with Down syndrome is reduced. This is not a diagnostic test," said Deborah A. Driscoll, M.D., immediate past chair of ACOG's Committee on Genetics.

First-trimester screening can also help detect other chromosomal abnormalities such as trisomy 18. In addition, measurement of NT may help detect pregnancies at risk for major heart defects in the fetus. However, first-trimester screening cannot be used as a screening test for spina bifida. According to the new ACOG Committee Opinion, sonographer training and ongoing quality assurance are essential if NT is used as a screening method. Since small differences in NT measurements can have a large impact on the risk prediction of Down syndrome, sonographers need to be monitored closely. ACOG does not recommend NT as a screening test for genetic abnormalities in singleton pregnancies since it has a low specificity. Although first-trimester screening is an option for some women, it should only be offered if the following criteria are met:

· Appropriate ultrasound training and ongoing quality monitoring programs are in place.

· There are sufficient information and resources to provide comprehensive counseling to women regarding the different screening options and limitations of these tests.

· Access to an appropriate diagnostic test is available when screening tests are positive.

"Women need to be aware of all the different screening options that are available, including their detection rates and limitations, so that they can choose the test that's best for them," said Dr. Driscoll.

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