Reference values for fetal limb biometry at 10-14 weeks of gestation

Objectives To calculate reference ranges for fetal limb measurements obtained by transabdominal ultrasound at 10-14 weeks of gestation. Methods Six hundred and six normal fetuses were examined transabdominally in a cross-sectional study by a single observer. The crown-rump length of the fetuses ranged from 31 to 78 mm. Measurement of the length of the humerus, ulna, femur, tibia and foot was attempted from the longest section of each structure. To assess intraobserver repeatability, three sets of repeated measurements were obtained in 26 fetuses. Results An appropriate ultrasound measurement was obtained in a percentage of cases ranging from 93.2% to 97.9%. A significant correlation was, found between crown-rump length measurements and humerus length (r=0.74, P<0.001), ulna length (r=0.70, P<0.001), femur length (r=0.77, P<0.001), tibia length (r=0.69, P<0.001) and foot length (r=0.58, P<0.001). Crown-rump length-specific reference ranges for each measurement were calculated with the method of scaled absolute residuals. The study of intraobserver variability showed coefficients of variation ranging from 7.9 to 10.0% and intraclass correlation coefficients ranging from 0.89 to 0.94. Conclusions Fetal limb size is strongly correlated with crown-rump length. Despite a significant biological variability of the measurements, the availability of reference ranges could be of help in the early diagnosis of fetal skeletal dysplasias.