ADJUSTING THE RISK FOR TRISOMY-21 ON THE BASIS OF 2ND-TRIMESTER ULTRASONOGRAPHY

OBJECTIVE: Our purpose was to establish the sensitivity and specificit y of various ultrasonographic markers of trisomy 21 in the second trim ester of pregnancy on the basis of literature review and to generate t ables that would allow adjusting the risk for trisomy 21, and therefor e the need for genetic amniocentesis, depending on the presence or abs ence of these markers. STUDY DESIGN: A computer search was performed o f the English literature, including the years 1983 through 1993, of st udies that used second-trimester ultrasonography to detect fetuses wit h trisomy 21. After statistical analysis of the reported studies was p erformed, the average sensitivity and specificity of the following ult rasonographic markers were determined: structural malformations, short femur, short humerus, combination of short femur and short humerus, p yelectasis, nuchal fold thickening, echogenic bowel, and short ear len gth. After the average sensitivity and specificity of these ultrasonog raphic markers were established, appropriate tables were generated by Bayes' theorem to adjust the risk for trisomy 21 in the second trimest er depending on the presence or absence of these markers. Statistical analyses were performed with the statistical package Excel on a person al computer. RESULTS: The average detection rate (sensitivity) of stru ctural fetal malformations was 28%, short femur 31%, short humerus 33% , short femur and humerus 32%, nuchal fold thickening 32%, echogenic b owel 7%, and short ear length 71%. The nuchal fold thickening had the highest specificity (99.5%). Isolated pyelectasis was not associated w ith an increased risk for trisomy 21. However the risk was increased w hen pyelectasis was associated with other markers. In the presence of normal ultrasonographic results, the negative prediction can be combin ed with maternal age-related or biochemical prediction of trisomy 21 t o help in the informed consent process in counseling women about the b enefits and harms of genetic amniocentesis. Genetic amniocentesis shou ld be considered in women of any age when second-trimester ultrasonogr aphy reveals the presence of one or more of the following: fetal struc tural malformations, short femur (determined by biparietal diameter-to -femur length ratio), combination of Short femur and humerus, abnormal (greater than or equal to 6 mm) nuchal fold thickening, echogenic bow el, or short ear length. CONCLUSION: In experienced hands second-trime ster ultrasonography may be used to adjust the priori risk of both hig h and low-risk women for trisomy 21 and therefore the need for genetic amniocentesis.