الفهرس 
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Abstract
EFBW is needed especially when head measurement is impossible, because the fetal head is positioned low in the pelvic brim. A convenient method for estimating fetal body weight without head measurement was thus required.
Isobe .,(2004) derived a formula from only thigh measurements using conventional two dimensional ultrasound .The newly derived formula was quite simple, involving only two thigh parameters without the need for head measurement.
The aim of this study was to assess the accuracy of newly established Isobe’s formula to estimate fetal body weight at third trimester using two thigh parameters; femur length ( FL ) and cross sectional area of the thigh ( CSAT ) .
This prospective study was performed at Ain Shams University Maternity Hospital between July 2010 and March 2011. It included 140 pregnant ladies in their third trimester, with singleton fetuses, with cephalic presentation without any structural abnormalities and normal amniotic fluid index, who are planned to deliver by elective cesarean section within 24 hours after having two dimensional ultrasound examinations.
All measurements were performed by an expert operator in the fetal medicine unit using a trans-abdominal ultrasound with 5.0 MHz convex probe (Mindray DP-6900). Parameters like Bi-parietal diameter (BPD), Abdominal Circumference (AC), Femur length (FL) and Cross sectional area of thigh (CSAT) had been taken respectively, amniotic fluid had been assessed using amniotic fluid index method by dividing the abdomen into four quadrants and measurement of amniotic fluid pockets in each quadrant.
The estimated fetal body weight was calculated twice as follow:
1-Using Hadlock’s formula, which had been calculated by the machine programmed software, using BPD, AC and FL.
2-Using Isobe’s formula, which had been calculated manually using FL and CSAT as follow:
EFBW = 13 × (FL × √CSAT) + 39 (gm).
N.B: FL by millimeter, CSAT by centimeter.
The birth weight (BW) of the infant was measured immediately after delivery. This prospective study was analyzed and evaluated by comparing the results of EFBW using the previously illustrated newly established Isobe’s formula using femur length (FL) and cross sectional area of the thigh (CSAT) and already established commonly used Hadlock formula using bi-parietal diameter (BPD), abdominal circumference (AC) and femur length (FL) and actual birth weight.
Actual birth weight was taken as the gold standard. Differences among estimated weights from the Isobe formula, Hadlock formula and the actual birth weight was assessed by a paired t test.
Results:
The mean actual birth weight in included women was 3169.5 ± 605.89 g. Of the 140 included neonates, 119 (85%) had average birth weight [2500 – 4000 g], 11 (7.9%) had low birth weight [<2500 g], while 10 (7.1%) had large birth weight [> 4000 g].
There was a significant positive correlation between actual birth weight and each of EFW using Hadlock’s formula, EFW using Isobe’s formula. The highest correlation coefficient was with EFW using Isobe’s formula [r=0.908, p˂0.001], indicating the most significant association.
The mean paired difference between EFW using Hadlock’s formula and actual birth weight was 90.29 ± 370.71 g [95% CI (28.33 to 152.23 g); p=0.005]. The mean paired difference between EFW using Isobe’s formula and actual birth weight was 60.21 ± 268.75 g [95% CI (15.31 to 105.21 g); p=0.009]. The narrower 95% CI and the higher p value for the Isobe’s formula when compared to Hadlock’s formula denote closer values of the estimated fetal weight to the actual birth weight using the former formula than the latter one. Of note, the mean paired difference was positive in both formulae (i.e. the error is toward overestimation).
Among the average birth weight category, the mean paired difference was lower when Isobe’s formula rather than Hadlock’s formula was used [63.11 ± 221.59 g vs. 111.18 ± 353.7 g]. The difference was positive in both formulae [denoting an overestimating error].
Among the low birth weight category, both formulae had mean paired differences markedly higher than those noticed among the average birth weight category, and it was noted that the Hadlock’s formula had lower mean paired difference when compared to the Isobe’s formula [270 ± 273.42 g vs. 434.55 ± 138.7 g]. The difference was positive in both formulae [denoting an overestimating error].
Among the large birth weight category, both formulae had mean paired differences markedly higher than those noticed among the average birth weight category, and it was noted that both Hadlock’s formula and Isobe’s formulae had comparable mean paired difference [-356 ± 363.78 g vs. -386 ± 240.19 g]. The difference was negative in both formulae [denoting an underestimating error].