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- Materials and Methods
Sonographic assessment of fetal growth for the estimation of fetal weight (EFW) is a common practice in obstetrics, providing valuable information for planning the mode of delivery and management of labor. Most formulae were proposed in the early 1980s using different combinations of standardized fetal biometric parameters, such as biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC) and femur length (FL)1. Unfortunately, the accuracy of EFW is compromised by significant intra- and interobserver variability, and many of the existing formulae are generally inaccurate at the extremes of fetal weight2. AC is widely recognized as the most useful dimension with which to evaluate fetal growth, although it is subject to larger variability compared with linear measurements3. None of these parameters, however, accounts for increased soft tissue mass, which leads to an underestimation of fetal weight1; this has been demonstrated in infants of diabetic mothers4, in whom the increased lean body mass was not estimated by standard measurements.
We have proposed the sonographic measurement of fetal mid-thigh soft tissue thickness (STT, involving adipose tissue plus lean mass) as a possible parameter for EFW5. This linear measurement ensures a good estimate of lean and fat mass of the fetal thigh. The aim or our present study was to evaluate the association between this new sonographic parameter and birth weight. We investigated technical aspects of this measurement to assess its reliability and reproducibility. Furthermore, we propose a new equation for the prediction of birth weight.
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- Materials and Methods
This multi-phase study confirms the potential of the linear measurement of mid-thigh STT as a valuable parameter in the sonographic assessment of fetal growth and EFW. This measurement is easy to make and has great reproducibility. Furthermore, the novel equation for EFW that we have proposed is apparently at least as reliable as the most widely used formulae for EFW.
It is intuitive that body weight derives from a combination of height as well as lean and fat mass. Several studies have been carried out to verify the ability of fetal fat tissue measurement to correctly predict fetal macrosomia11, 16–18, and sonographic evaluation of fetal fat mass has been proposed as a method for assessing fetal overgrowth19. In contrast, very few studies have been published on fetal lean mass20, 21. Reduced lean mass was demonstrated in fetuses with growth restriction22, which may explain the increasing error of EFW in these cases2.
The assessment of adipose tissue in fetal limbs to evaluate fetal growth and EFW has been investigated previously. Santolaya-Forgas et al.11 proposed the fetal subcutaneous tissue/FL ratio as a novel parameter for identification of large-for-gestational-age fetuses (involving the measurement of subcuticular adipose tissue only). Subsequently, the introduction of three-dimensional ultrasound has led some authors to propose new formulae that incorporate volumetric data from fetal limbs. Lee et al.23, 24 utilized 3D ultrasound to derive fractional arm and thigh volumes as fetal soft tissue parameters for assessment of growth and weight estimation. Although the results showed potential, the application of this technique is limited by the excessive time required (for scanning and data processing) and by the need for a 3D ultrasound machine and specific software. Nevertheless, these findings further support the potential of measuring soft mass (fat and lean mass) in the assessment of fetal growth and EFW.
Here, we have proposed a novel approach for the measurement of fetal soft mass. The linear measurement of the tissue above the external side of the fetal femur provides an easy and straightforward method with which to assess the amount of fat and muscular mass of the fetal thigh.
The deviation of EFW from actual birth weight is due to a combination, in approximately equal proportions, of measurement error and the intrinsic properties of the formula25. Most sonographic formulae for EFW are based on multiple fetal measurements1. Formulae combining more than two parameters are deemed more reliable than are those with one or two measurements26, although a clear explanation for this has not yet been proposed. From a purely statistical point of view, the presence of different variables in a formula increases the risk of multicollinearity and enhances the internal error of each measurement.
Clinically, the proposed formula can be of practical use in situations in which head measurements cannot be taken properly due to fetal head engagement. In contrast, we excluded from this study all breech presentations, in which the fetal hip can be down into the pelvis, causing distortion of the thigh profile. However, such cases represent only a small percentage of all deliveries. Furthermore, compared with circumferences, linear measurements are more easily taken by obstetricians/midwives who have little experience in sonography27, and linear measurements of fetal biometry are more reproducible than are circumferences3. Of the standard sections for fetal biometry, the view for FL measurement is probably the easiest image for non-expert sonographers to obtain adequately28, while quality images for AC measurement are less easily obtained, even for operators with some experience27. Measurements made from suboptimal images contribute to interobserver variability and are a major bias in EFW27.
The limitations of this study must be considered. The population consisted of Caucasian women with singleton cephalic pregnancies and evidence of normal amniotic fluid volume. Strict statistical and mathematical approaches were used to test STT as a possible new parameter for the assessment of fetal growth and estimation of birth weight. Our findings are promising, but larger studies are required to confirm the reliability of this novel parameter and the accuracy of the proposed formula for EFW under different circumstances, such as breech presentation, oligohydramnios and fetal growth alterations.