Usefulness of a short femur in the in utero detection of skeletal dysplasias.

In 28 fetuses studied during a 4 1/2-year period, the initial femur was below 2 standard deviations (SDs) of the mean when compared with the biparietal diameter. These fetuses were considered at risk for skeletal dysplasias and were followed up. Studies were performed at a mean gestational age of 26.7 weeks (range, 15.3-41.0 weeks). Group 1 had a femur length 1-4 mm below the 2-SDs line (range, -2.0 to -4.0 SDs); no other abnormalities were detected. Interval examination of 12 femurs showed that 10 either remained shortened to the same degree or had a growth spurt. At birth, all subjects were healthy except one with mild growth retardation and one with a chromosomal abnormality. Of the two subjects that failed to continue normal growth, one was healthy and the other was a heterozygous achondroplastic dwarf. Group 2 had greater femoral shortness; all measurements were more than 5 mm below the 2-SD line (range, -4.3 to -31.0 SDs). All had fetal abnormalities and significant skeletal dysplasias. The authors conclude that the number of millimeters below the 2-SDs line is an accurate, easy criterion for evaluation of femoral length.