Ultrasonographic fetal thigh measurement in the estimation of fetal weight based on Isobe's formula in women with an engaged fetal head in the pelvis: a comparative study.

Abstract

Because of the inaccuracy of the biparietal diameter in cases with an engaged fetal head in the pelvis, measuring the estimated fetal weigh (EFW) using Hadlock's formula could be compromised in these cases. The aim of this prospective study is to determine the accuracy of using only two thigh parameters, the femur length (FL) and the cross-sectional area of the thigh (CSAT) (Isobe's formula), in detecting the fetal weight of both engaged pelvis fetuses and non-engaged head fetuses and to compare this method with Hadlock's formula in both groups using the actual birth weight as a gold standard. The study included 51 cases with an engaged fetal head and 51 cases with a non-engaged fetal head that came in active labour. 2D ultrasonography examination was performed to determine the EFW using both Hadlock's formula and Isobe's formula. The EFW was then compared with the actual birth weight after delivery. There was a strong positive correlation between Isobe's formula and the actual birth weight in the engaged fetal head group (r = 0.993, p < 0.01), but there was a strong positive correlation between Hadlock's formula and the actual birth weight in the non-engaged fetal head group (r = 0.994, p < 0.01). We concluded that Isobe's formula is convenient in predicting the fetal weight, especially when head measurements are difficult to assess (in the engaged fetal head group). It can be used with 2D ultrasonography as an alternative to Hadlock's formula in cases with an engaged fetal head in the pelvis.