Influence of fetal growth patterns on sonographic estimation of fetal weight

Abstract

Six published fetal weight estimating regression models proposed for clinical use were evaluated in 259 pregnant women who delivered within 72 h of an ultrasound evaluation performed with sector scanner. The patient sample included 89 (33.2%) fetal weights that were below the 10th or above the 90th percentile for menstrual age. The actual mean percent error (systematic error), standard deviation (random error), and the number of large errors of prediction for all equations were greatest in fetuses that were small- and large-for-gestational age. Whereas there were no significant differences between equations for the patient sample as a whole, equation AC,BPD (Shepard) had the smallest systematic error in intrauterine growth retarded, premature, and normal-term fetuses less than 4000 g. Conversely, the systematic error of the models that included femur length was smallest at the upper end of the weight scale and in macrosomic fetuses in general. In that regard, the accuracy of fetal weight prediction could be increased by selecting the appropriate model for the proper clinical indications. Although these findings can be explained by the limitations of the current regression models in estimating fetal soft tissue mass, a subtle effect of the use of the sector scanner on the results of this study cannot be completely excluded and requires further investigation.

Influence of ultrasound‐to‐delivery interval and maternal–fetal characteristics on validity of estimated fetal weight

“…With a mean percentage difference close to 0% and a SD of 9%, our results are consistent with the findings of previous studies 7,15,20,21 . Most validation studies have failed to demonstrate substantial systematic bias in weight estimates obtained with one of the Hadlock formulae among a sample with a wide range of birth weights.…”

“…Some studies have observed SDs in percentage differences as low as 7-8% 7,20 , whereas others have found larger variations. For example, Simon et al noted a SD of 11.7% among estimates obtained with the Hadlock formula based on four measurements 15 . We observed that the most reliable EFWs were obtained among deliveries with birth weights of 3000-3999 g. Consistent with previous studies 7,12,15 , we found a tendency to overestimate the weight of small fetuses (< 3000 g) and underestimate the weight of large fetuses (≥ 4000 g), with the effect most pronounced for fetuses ≥ 4500 g. Hadlock and others have found the effects to be of similar magnitude in either direction 7,12,15 .…”

“…For example, Simon et al noted a SD of 11.7% among estimates obtained with the Hadlock formula based on four measurements 15 . We observed that the most reliable EFWs were obtained among deliveries with birth weights of 3000-3999 g. Consistent with previous studies 7,12,15 , we found a tendency to overestimate the weight of small fetuses (< 3000 g) and underestimate the weight of large fetuses (≥ 4000 g), with the effect most pronounced for fetuses ≥ 4500 g. Hadlock and others have found the effects to be of similar magnitude in either direction 7,12,15 . The results of Kurmanavicius et al, however, were consistent with ours 13 ; they found percentage errors close to zero below 3500 g and increasingly negative mean percentage errors beyond 3500 g. Interestingly, when we examined progressively smaller birth weight groups in our study, we observed a trend toward increasing overestimation.…”

Estimation of Fetal Weight and Its Correlation With Actual Birth Weight by Sonographic Measurement of Fetal Abdominal Subcutaneous Tissue Thickness (Fastt)

“…In study conducted by Simon et al (1987), foetal weight prediction was less accurate in small and large fetuses and particularly when there were deviations from normal growth pattern. The Shepard equation (AC, BPD) performed better in intrauterine growth retarded, premature and normal term fetuses less than 4000 gm at least as far as the magnitude of the systematic error was concerned.…”

“…In contrast the FL models were more accurate in large fetuses and in macrosomic fetuses in general. 10 In 1982, William ultrasonically calculated fetal weight by a formula that utilized biparietal diameter and abdominal circumference. 8 Log birth weight = 1.7492 + 0.166 x BPD + 0.046AC -2, 646 (BPD x AC) 100 In a study by Hadlock et al, addition of femur length to head and abdomen measurements increases the accuracy of in utero weight estimates based on ultrasound studies.…”