Normal fetal pelvis: Important factors for morphometric characterization with US

PURPOSE: To prospectively evaluate iliac angle and iliac length in a large number of normal fetuses and to identify factors that may influence these m easurements. MATERIALS AND METHODS: At antenatal ultrasonography (US) in 356 fetuses, th e iliac angle and iliac length were measured at two axial levels (superior and inferior). In mixed linear models,the statistical significance and magn itude of effect on the measurement of iliac angle and iliac length were est imated for gestational age, fetal sex, maternal diabetes status, axial leve l, and spine position relative to the transducer. RESULTS: Statistically significant effects were found for gestational age, axial level, and spire orientation but not for fetal sex or maternal diabet es status. The iliac angle was found to decrease by 15.7 degrees from the s uperior to inferior portion of the pelvis, decrease by approximately 0.37 d egrees/wk, and decrease by as much as 15.6 degrees when the spine is direct ed to the side. Iliac length was found to increase by 0.8 mm/wk from 13 wee ks to term, decrease by 1.2 mm from the superior to the inferior portion of the pelvis, and increase by as much as 1.29 mm when the spine is not direc tly subjacent to the transducer. CONCLUSION: The axial level of measurement, gestational age, and spine orie ntation must be accounted for if these morphometric indexes are used to dis criminate fetuses with and those without Down syndrome.