Intrapartum fetal subcutaneous tissue/femur length ratio: an ultrasonographic clue to fetal macrosomia.

OBJECTIVE: Our purpose was to compare the accuracy of ultrasonographic intrapartum measurement of fetal abdominal circumference, estimated fetal weight, and fetal subcutaneous tissue/femur length ratio in predicting large-for-gestational-age fetuses.

STUDY DESIGN: A cross-sectional analysis of 173 normal patients delivered of normal, appropriate-for-gestational-age infants (group 1) was performed to determine the normal changes of standard fetal biometric parameters and subcutaneous tissue throughout pregnancy. Measurements of fetal subcutaneous tissue were made at the level of the femoral diaphysis. A second group of 101 well-dated patients had these measurements obtained within 24 hours of delivery (group 2). Large for gestational age was defined as a birth weight > 90th percentile for gestational age. The sensitivity and specificity for prediction of large for gestational age of an intrapartum measurement of an abdominal circumference > 90th percentile, estimated fetal weight > 90th percentile, and fetal subcutaneous tissue/femur length ratio > 2 deviations of the mean was calculated.

RESULTS: In group 1 the mean gestational age was 31.4 +/- 5.4 weeks (range 17 to 41 weeks). The femur length, abdominal circumference, and fetal subcutaneous tissue correlated well with gestational age (p < 0.0001). The fetal subcutaneous tissue/femur length ratio was stable throughout pregnancy, with a mean of 0.05 +/- 0.014 (range 0.02 to 0.09, R2 0.09). In group 2 mean gestational age was 38 +/- 2.5 weeks (range 30 to 42 weeks). Mean birth weight was 3280 +/- 740 gm (range 1513 to 4801 gm). Nineteen (19%) fetuses were large for gestational age. Significant differences were found between the appropriate- and large-for-gestational-age fetuses for birth weight, abdominal circumference, estimated fetal weight, and fetal subcutaneous tissue/femur length ratio (p < 0.0001). The sensitivity and specificity of the fetal abdominal circumference, estimated fetal weight, and fetal subcutaneous tissue/femur length ratio were 44% and 98%, 68% and 85%, and 82% and 96%, respectively.

CONCLUSION: The fetal subcutaneous tissue/femur length ratio is a gestational age-independent parameter that has a greater sensitivity than the fetal abdominal circumference or estimated fetal weight formula for the intrapartum identification of large-for-gestational-age fetuses.