A new method for fetal weight estimation using real-time ultrasound

Does the Porter formula hold its promise? A weight estimation formula for macrosomic fetuses put to the test

Purpose

Estimating fetal weight using ultrasound measurements is an essential task in obstetrics departments. Most of the commonly used weight estimation formulas underestimate fetal weight when the actual birthweight exceeds 4000 g. Porter et al. published a specially designed formula in an attempt to improve detection rates for such macrosomic infants. In this study, we question the usefulness of the Porter formula in clinical practice and draw attention to some critical issues concerning the derivation of specialized formulas of this type.

Methods

A retrospective cohort study was carried out, including 4654 singleton pregnancies with a birthweight ≥ 3500 g, with ultrasound examinations performed within 14 days before delivery. Fetal weight estimations derived using the Porter and Hadlock formulas were compared.

Results

Of the macrosomic infants, 27.08% were identified by the Hadlock formula, with a false-positive rate of 4.60%. All macrosomic fetuses were detected using the Porter formula, with a false-positive rate of 100%; 99.96% of all weight estimations using the Porter formula fell within a range of 4300 g ± 10%. The Porter formula only provides macrosomic estimates.

Conclusions

The Porter formula does not succeed in distinguishing macrosomic from normal-weight fetuses. High-risk fetuses with a birthweight ≥ 4500 g in particular are not detected more precisely than with the Hadlock formula. For these reasons, we believe that the Porter formula should not be used in clinical practice. Newly derived weight estimation formulas for macrosomic fetuses must not be based solely on a macrosomic data set.

Electronic supplementary material

The online version of this article (10.1007/s00404-019-05410-7) contains supplementary material, which is available to authorized users.

Role of fetal length in the prediction of fetal weight

OBJECTIVES

The purpose of this study was to compare the accuracy of routine fetal biometric indices in the prediction of fetal length and to determine whether more accurate sonographic measures of fetal length can improve the accuracy of fetal weight estimation.

METHODS

The accuracy of the common sonographic fetal biometric indices for predicting fetal length was determined using 3689 sonographic weight estimations performed within 3 days before delivery. The fetal length at the time of the sonographic examination was assumed to be equal to the neonatal length, which is routinely measured within 24 hours of delivery. Two new regression models for fetal weight estimation, one with and one without fetal length as an independent variable, were generated to determine the potential contribution of more accurate predictors of fetal length to the accuracy of fetal weight estimation.

RESULTS

Abdominal circumference was a significantly more accurate predictor of fetal length (r = 0.732) compared with femur length (r = 0.712), biparietal diameter (r = 0.644), and head circumference (r = 0.661; P < .05), although each of these biometric indices explained only about 50% of the variance in fetal length (R(2) = 0.423-0.548). The addition of fetal length as an independent variable to a birth weight prediction model significantly improved the model's correlation with birth weight(r = 0.917 versus 0.903; P = .006), systematic error (0.2% versus 0.6%; P < .001), random error (6.7% versus 7.5%; P < .001), mean absolute percent error, and the proportion of estimations within 5% and 10% of birth weight.

CONCLUSIONS

The correlation between routine biometric indices and fetal length is limited. Identification of new fetal sonographic biometric indices with greater predictive accuracy for fetal length may improve the accuracy of fetal weight estimation.

A systematic review of the ultrasound estimation of fetal weight

OBJECTIVES

The range and use of ultrasound fetal measurements have gradually been extended. Measurements have been combined to estimate fetal weight by mathematically based non-linear regression analysis or physically based volumetric methods. Fetal weight estimation is inaccurate, with poor sensitivity for prediction of fetal compromise. Several authors have shown the unacceptable level of intra- and interobserver variability in fetal measurement and the impact of errors on growth assessment. The aims of this study were to review the available methods and possible sources of inaccuracy.

METHODS

Four databases were searched for studies comparing ultrasound estimated fetal weight (EFW) with birth weight. Studies meeting the inclusion criteria evaluated 11 different methods. Errors were graphically summarized.

RESULTS

No consistently superior method has emerged. Volumetric methods provide some theoretical advantages. Random errors are large and must be reduced if clinical errors are to be avoided.

CONCLUSIONS

The accuracy of EFW is compromised by large intra- and interobserver variability. Efforts must be made to minimize this variability if EFW is to be clinically useful. This may be achieved through averaging of multiple measurements, improvements in image quality, uniform calibration of equipment, careful design and refinement of measurement methods, acknowledgment that there is a long learning curve, and regular audit of measurement quality. Further work to improve the universal validity and accuracy of fetal weight estimation formulae is also required.

The importance of quality management in fetal measurement

OBJECTIVES

The aims of this study were to evaluate factors contributing to inaccuracy in fetal measurements and to assess the clinical importance of measurement quality.

METHODS

One hundred images of biparietal diameter (BPD), head circumference (HC) and abdominal circumference (AC) measurements were collected from six centers (1800 measurements); the proportion meeting quality criteria was assessed. Four hundred images of AC were collected from one center, each image measured by ellipse fitting and tracing methods; clinical agreement between the methods was assessed. Fetal weight estimation (EFW) errors were compared between quality controlled and non-quality controlled studies. Images of three ACs on each of 400 fetuses were collected; where one measurement failed to meet quality criteria, it was compared with an optimal measurement on the same fetus.

RESULTS

Eighty-nine percent, 87% and 60% of BPD, HC and AC, respectively, met all quality criteria. Limits of agreement between ellipse and traced AC were -4.7 mm to 12.5 mm; 22% of sections were non-elliptical. EFW errors were significantly different but were confounded by differences in time to delivery. Limits of agreement between optimal and suboptimal AC measurements were -15.1 mm to 7.7 mm.

CONCLUSIONS

AC quality criteria are less easily recognized and obtained than those for head measurements; training, adherence to protocols and audit are important. Differences between ellipse and traced AC may not justify the use of separate charts; the number of non-elliptical sections suggests that ellipse fitting is not appropriate. Comparison between EFW errors is not a suitable tool for audit. Failure to meet quality criteria results in clinically significant errors.

Sonographic diagnosis of intrauterine growth restriction

Estimation of fetal weight in utero using multiple ultrasonic parameters remains the mainstay in screening for IUGR. The use of various fetal morphometric ratios and/or measurements of other fetal parameters may provide additional useful information. Serial evaluation to assess interval growth may be necessary to clarify the diagnosis. The use of Doppler ultrasound, especially the evaluation of the umbilical artery and middle cerebral artery velocity flow, is an important adjunct for both the diagnosis of IUGR caused by uteroplacental insufficiency and its continued management.

Selection of appropriate ultrasound methods for the estimation of fetal weight

Many methods for fetal weight estimation have been proposed. Earlier formulae were derived solely by regression analysis but more recently formulae based on physical models have been developed. Some of these formulae are lengthy and difficult to use. The main purpose of this study was to evaluate a concise area based formula developed several years ago against popular and recently published formulae. Data from 388 patients over a 5 year period were collected from clinically required ultrasound scans performed within 10 days of delivery. Four of the seven formulae evaluated proved to be equally accurate over a broad range of birthweights (mean absolute deviations 245-249 g), including the area based formula and a circumference based formula derived from it. On the grounds of the number of terms, the precision of coefficients, the physical meaning and the greater accuracy of individual area measurements compared with those derived from circumference, the area based formula should be the method of choice.

Fetal weight estimation in twin pregnancies

In this study the fetal weight was estimated by ultrasound measurements of the biparietal (BPD) and abdominal diameters (AD) in 154 consecutive twin pregnancies. In 80 twins an attempt to estimate fetal weight was made 0-4 days before delivery (26% of all twins examined). In 84% (67 infants) it was possible to estimate the fetal weight using the formula (0.0108 X AD1.72 X BPD0.99). 60% of the birth weights deviated less than 10% from the estimated fetal weight and 83% deviated less than 15%. The prediction error was nearly constant, expressed as percent of actual weight in the different weight groups (8.5-9.8%). In 19 LGA infants, 69% of the birth weights deviated less than 10% from the estimated fetal weight and 70% deviated less than 15%.