Sonographic screening for fetal aneuploidy: first trimester.

Screening for fetal aneuploidy is now possible during the first trimester using sonographic and biochemical markers. The aim of this review was to summarize the efficacy and use of nuchal translucency in screening for fetal aneuploidy, especially fetal Down syndrome, and other anomalies. We reviewed available literature regarding first‐trimester screening. This includes more than 16 studies of nuchal translucency as a marker for fetal aneuploidy published since 1995. Although early studies showed wide variation in detection of fetal Down syndrome when using nuchal translucency, more recent studies showed sensitivities of approximately 70% to 80%, for a 5% false‐positive rate. Increased nuchal translucency has also been found to be a marker for other aneuploidies, including trisomy 18, trisomy 13, and Turner syndrome. Maternal serum biochemical screening can be used as a test for aneuploidy during the first trimester The 2 maternal serum markers that appear to be most useful in the late first trimester are the free beta subunit of human chorionic gonadotropin and pregnancy‐associated plasma protein A. Together with maternal age, these markers yield a detection rate for trisomy 21 of approximately 60%, for a 5% false‐positive rate. Because sonographic and biochemical markers appear to be largely independent, their combined risk results in improved detection rates compared with either method alone. As a result, the combination of nuchal translucency, biochemical markers, and maternal age has achieved a detection rate of approximately 85%, for a 5% false‐positive level for detection of trisomy 21. A newly proposed "integrated" approach using a panel of first‐ and second‐trimester markers suggests that further improvement in the screening performance is possible. A number of questions regarding first‐trimester screening remain. We address some of these questions: is first‐trimester screening more effective than second‐trimester screening? How to account for intrauterine lethality? Is earlier diagnosis important, and will it be accepted by patients? Is first‐trimester screening cost‐effective? How should first‐trimester screening be interpreted with second‐trimester tests? Despite encouraging data and general enthusiasm for first‐trimester screening for fetal Down syndrome and other aneuploidies, a number of questions remain about its implementation in the United States. Multicenter studies currently under way should help answer some of these questions.