Reevaluating humeral length for the detection of fetal trisomy 21

The purpose of this study was to analyze humeral length (HL) in a normal population and to compare that with HL in a population of fetuses with trisomy 21 to determine the most efficient discriminating parameters for diagnostic accuracy.

A nested case-control study comparing HLs from a normal population and a population of fetuses with trisomy 21 was conducted. Humeral length was regressed against gestational age for a consecutive well-dated population of normal singleton gestations presenting to the Washington University School of Medicine prenatal diagnosis units over a 5-year period. A second population of well-dated pregnancies with trisomy 21 diagnosed either prenatally or postnatally was also selected on the basis of the same criteria, except that anomalous fetuses were included. Various discriminating thresholds for a short HL were compared for efficiency in the detection of trisomy 21. These included the following: observed/expected HL (

A total of 620 normal pregnancies and 32 with trisomy 21 were extracted from the database. A receiver operating characteristic curve revealed HL less than the fifth percentile as the optimal discriminator for trisomy 21 detection (area under the receiver operating characteristic curve = 0.80). The positive likelihood ratio (LR+) was greatest (25.0) with HL less than the fifth percentile. When HL was considered in isolation without other sonographic markers of trisomy 21, the LR+ was 6.3.

Humeral length less than the fifth percentile was the most effective discriminator among the many studied.