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 f etal weight, and fetal subcutaneous tissue/femur length ratio in predi cting large-for-gestational-age fetuses. STUDY DESIGN: A cross-section al analysis of 173 normal patients delivered of normal, appropriate-fo r-gestational-age infants (group 1) was performed to determine the nor mal changes of standard fetal biometric parameters and subcutaneous ti ssue throughout pregnancy. Measurements of fetal subcutaneous tissue w ere 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 spec ificity 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 l ength ratio > 2 deviations of the mean was calculated. RESULTS: In gro up 1 the mean gestational age was 31.4 +/- 5.4 weeks (range 17 to 41 w eeks). The femur length, abdominal circumference, and fetal subcutaneo us tissue correlated well with gestational age (p < 0.0001). The fetal subcutaneous tissue/femur length ratio was stable throughout pregnanc y, with a mean of 0.05 +/- 0.014 (range 0.02 to 0.09, R(2) 0.09). In g roup 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). Nine teen (19%) fetuses were large for gestational age. Significant differe nces were found between the appropriate- and large-for-gestational-age fetuses for birth weight, abdominal circumference, estimated fetal we ight, and fetal subcutaneous tissue/femur length ratio (p < 0.0001). T he sensitivity and specificity of the fetal abdominal circumference, e stimated fetal weight, and fetal subcutaneous tissue/femur length rati o were 44% and 98%, 68% and 85%, and 82% and 96%, respectively. CONCLU SION: The fetal subcutaneous tissue/femur length ratio is a gestationa l age-independent parameter that has a greater sensitivity than the fe tal abdominal circumference or estimated fetal weight formula for the intrapartum identification of large-for-gestational-age fetuses.