SONOGRAPHIC SCREENING FOR TRISOMY-21 - FETAL HUMERUS-FOOT LENGTH RATIO, A USEFUL NEW MARKER

We have analyzed morphometric measurements from midgestational fetal n ecropsies and shown that arm and foot lengths are linear relationships versus gestational age (GA). Using foot length as the GA determinant, we found that the ratio of arm: foot length was also a linear relatio nship and was decreased in trisomy 21 fetuses when compared to age-mat ched normals. Based on these laboratory findings, we prospectively eva luated the use of the humerus:foot length ratio as a sonographic scree ning tool for identification of fetuses at risk for trisomy 21. Humeru s length, foot length and the humerus:foot length ratio were found to be linear relationships vs. gestational age in both the normal and tri somy 21 populations. However, the regressions for the humerus:foot len gth ratio were significantly different between normals and Down's fetu ses (p<0.001). We found that a humerus:foot length ratio less than or equal to 0.85 correctly identified 47% of our trisomy 21 fetuses (spec = 0.92, PPV = 0.25, NPV = 0.97). When compared to women greater than or equal to 35 years old in our high risk population, a humerus:foot l ength ratio less than or equal to 0.85 carried an odds ratio of 52.7 ( 99% CL = 9.72-285.23) for trisomy 21.