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Am Fam Physician. 2004;70(8):1592

First-Trimester Screening for Genetic Defects

The Committee on Genetics of the American College of Obstetricians and Gynecologists (ACOG) has issued a new report entitled, “ACOG Committee Opinion No. 296: First-Trimester Screening for Fetal Aneuploidy.” The report appears in the July 2004 issue of Obstetrics and Gynecology.

According to the report, first-trimester screening for genetic defects is now an option for pregnant women, but only if certain criteria are met. 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 detection rates comparable with 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 to 12 weeks of gestation 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 provides women the option to terminate the pregnancy earlier, which is associated with reduced maternal morbidity.

During 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 of gestation. The newer first-trimester screening method includes measurement of NT, free beta subunit of human chorionic gonadotropin (β-hCG), and pregnancy-associated plasma protein-A (PAPP-A). It has a detection rate for Down syndrome comparable with the more commonly used second-trimester screening using four serum markers (alpha-fetoprotein, β-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 because screening tests can give false-positive results.

First-trimester screening also can 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.

Sonographer training and ongoing quality assurance are essential if NT is used as a screening method. Because 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 using the NT measurement by itself to screen for Down syndrome because it has a high positive screen rate when used without serum markers. 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.

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