A review of sonographic estimate of fetal weight: Vagaries of accuracy

Overestimation of fetal weight by ultrasound: does it influence the likelihood of cesarean delivery for labor arrest?

OBJECTIVE

We sought to determine whether the overestimation of ultrasound-derived estimated fetal weight (EFW) is associated with increased diagnosis of labor arrest.

STUDY DESIGN

This is a historical cohort study of nulliparous women with term pregnancies who underwent bedside ultrasound examination for EFW before labor induction. Labor outcomes of women with EFW overestimation > 15% the actual birthweight were compared with those with EFW not overestimated.

RESULTS

Overestimation of EFW occurred in 9.5% of cases (23/241). The rate of cesarean delivery (CD) for labor arrest was higher for those with EFW overestimation (34.8% vs 13.3%; P = .01) even though there were no differences in length of the induction duration. After adjusting for confounding factors, EFW overestimation remained associated with CD for labor arrest (odds ratio, 4.8; 95% confidence interval, 1.5-15.2).

CONCLUSION

Our finding suggests that an overestimation of EFW may be associated with a lower threshold for CD for labor arrest.

New regression formulas for sonographic weight estimation within 10, 7, and 3 days of delivery

OBJECTIVES

The purpose of this study was to develop new regression formulas based on large numbers of sonographic examinations performed within 10, 7, and 3 days of delivery.

METHODS

Sonographic fetal biometric measurements and delivery ward data for an unselected population were analyzed. Multivariate linear regression models were fitted to the sonographic data to predict the actual birth weight (BW) within 10, 7, and 3 days.

RESULTS

The analyses included 6289, 5449, and 4007 patients who underwent sonographic examinations within 10, 7, and 3 days of delivery, respectively. All models yielded very high correlation coefficients (r = 0.927-0.958; R(2) = 0.859-0.918), low mean deviations between the calculated and actual BWs (6.4%-6.6% +/- 1 SD of 5.5%-5.9%), and high percentages of the calculated BW within 10% of the actual BW (78.5%-80.4%). Estimated fetal weight analyses made within 3 days of delivery yielded slightly better results than within 7 and 10 days.

CONCLUSIONS

The new regression formulas yielded overall similar results, with a small advantage for estimates calculated within 3 days of delivery. Further prospective studies are needed to compare the accuracy of these formulas with those used to date.

The epidemiology of neonatal brachial plexus palsy in the United States

BACKGROUND

The nationwide incidence of neonatal brachial plexus palsy in the United States is unknown. The purpose of this study was to determine the incidence of this condition in the United States and to identify potential risk factors for neonatal brachial plexus palsy.

METHODS

Data from the 1997, 2000, and 2003 Kids' Inpatient Database data sets were utilized for this study. Patients were identified with use of the International Classification of Diseases, Ninth Revision (ICD-9), code 767.6 for neonatal brachial plexus palsy. Previously reported risk factors for this condition, including shoulder dystocia, instrumented delivery, breech delivery, an exceptionally large baby (>4.5 kg), heavy infant weight for gestational dates, multiple birth mates, and cesarean delivery, were also identified with use of ICD-9 codes. Multivariate logistic regression analysis was utilized to assess the association of neonatal brachial plexus palsy with its risk factors, after adjusting for sociodemographic characteristics, such as gender, race, and payer status; hospital-based characteristics, such as number of hospital beds, hospital location, region, type, and teaching status; and the effect of time.

RESULTS

Over eleven million births were recorded in the database, and 17,334 had a documented brachial plexus injury in the total of three years, yielding a nationwide mean and standard error of incidence of neonatal brachial plexus palsy in the United States of at least 1.51 +/- 0.02 cases per 1000 live births. The incidence of this condition has shown a significant decrease over the years (p < 0.01). In the multivariate analysis, shoulder dystocia had a 100 times greater risk, an exceptionally large baby (>4.5 kg) had a fourteen times greater risk, and forceps delivery had a nine times greater risk for injury. Having a twin or multiple birth mates and delivery by cesarean section had a protective effect against the occurrence of neonatal brachial plexus palsy. Forty-six percent of all children with neonatal brachial plexus palsy had one or more known risk factors, and fifty-four percent had no known risk factors.

CONCLUSIONS

This nationwide study of neonatal brachial plexus palsy in the United States demonstrates a decreasing incidence over time. Shoulder dystocia poses the greatest risk for brachial plexus injury, and having a twin or multiple birth mates and delivery by cesarean section are associated with a protective effect against injury. Most children with neonatal brachial plexus palsy did not have known risk factors.

An informative probability model enhancing real time echobiometry to improve fetal weight estimation accuracy

A multinormal probability model is proposed to correct human errors in fetal echobiometry and improve the estimation of fetal weight (EFW). Model parameters were designed to depend on major pregnancy data and were estimated through feed-forward artificial neural networks (ANNs). Data from 4075 women in labour were used for training and testing ANNs. The model was implemented numerically to provide EFW together with probabilities of congruence among measured echobiometric parameters. It enabled ultrasound measurement errors to be real-time checked and corrected interactively. The software was useful for training medical staff and standardizing measurement procedures. It provided multiple statistical data on fetal morphometry and aid for clinical decisions. A clinical protocol for testing the system ability to detect measurement errors was conducted with 61 women in the last week of pregnancy. It led to decisive improvements in EFW accuracy.

A computerized method for accurately predicting fetal macrosomia up to 11 weeks before delivery

OBJECTIVE

To improve the prediction of birth weight and fetal macrosomia by combining sonographically derived fetal biometric data with routinely recorded pregnancy-specific information.

STUDY DESIGN

Retrospective data were obtained for 218 normal gravidas who had obstetrical ultrasonography performed within 11 weeks of delivery. Multiple regression was employed to derive a set of equations for predicting birth weight that used different combinations of ultrasonographic and pregnancy-specific variables.

RESULTS

A set of 38 unique combination equations was derived to accurately predict birth weight up to 11 weeks before delivery. The equations use different combinations of ultrasonographic and pregnancy-specific variables, so that predictions are still possible in the face of missing data. When ultrasonographic measurements are taken within 3 weeks of delivery, fetal macrosomia is predicted with 75% sensitivity, 93% specificity, and 67% and 95% positive and negative predictive value, respectively. The equations are equally as accurate for primiparous and multiparous women from all racial groups. A jackknifing procedure was used to validate the predictive accuracy of the equations for use with new subjects.

CONCLUSION

The combined approach of predicting fetal macrosomia using ultrasonographic fetal measurements and pregnancy-specific characteristics is superior to pre-existing approaches that rely on either method alone. The method can be used up to 11 weeks before delivery, allowing fetal macrosomia to be predicted reliably in low-risk populations sufficiently early for prospective clinical intervention to be undertaken.

Estimation of fetal weight: reference range at 20-36 weeks' gestation and comparison with actual birth-weight reference range

OBJECTIVES

To formulate reference charts and equations for estimated fetal weight (EFW) from a large sample of fetuses and to compare these charts and equations with those obtained for birth weight during the same study period and in the same single health authority.

METHODS

Biometric data were obtained at 20-36 weeks' gestation from routine screening examinations spanning 4 years. Exclusion criteria were a known abnormal karyotype or congenital malformation and multiple pregnancy. No data were excluded on the basis of abnormal biometry. EFW was calculated based on Hadlock's formula. We used a polynomial regression approach (mean and SD model) to compute a new reference chart for EFW. This chart was compared with that of birth weight at 25-36 weeks' gestation during the same study period and in the same health authority.

RESULTS

18,959 fetuses were included in the study. New charts and equations for Z-score calculations at 20-36 weeks' gestation are reported. Comparison with the birth-weight chart showed that the EFW was noticeably larger at 25-36 weeks' gestation. At 28-32 weeks' gestation, the 50th centile for birth weight compared approximately with the 10th centile for EFW.

CONCLUSION

We present new reference charts and equations for EFW. EFW is computed throughout gestation based on measurements in healthy fetuses. However, before full term, birth-weight charts reflect a significant proportion of growth-restricted fetuses that deliver prematurely. We provide additional evidence that comparing EFW with birth-weight charts is misleading.

Sonographic estimate of birth weight among high-risk patients: feasibility and factors influencing accuracy

OBJECTIVE

This study was undertaken to determine the feasibility of detecting abnormal fetal growth among patients undergoing biophysical profile (BPP) and to identify the factors those influence the accuracy.

STUDY DESIGN

Retrospectively singletons with reliable gestational age (GA) having a BPP were identified. Fetal growth restriction (FGR) and large-for-gestational age (LGA) were based on estimated or actual birth weight 10% or less or 90% or greater for GA, respectively. Likelihood ratio (LR), odds ratio (OR) and 95% CIs were calculated and multivariate predictive models used.

RESULTS

Among the 1934 consecutive patients that met the inclusion criteria, the LR of detecting FGR was 10.9 and of LGA, 17.4. Multivariate analysis indicates that accurate classification of fetal growth is significantly better with hydramnios (OR 1.78, 95% CI 2.68), if the GA is less than 32 weeks (OR 3.71, 95% CI 1.50-9.16) or GA is between 32.1 and 36.9 weeks (OR 1.43, 95% CI 1.05-1.96).

CONCLUSION

It is feasible to accurately identify abnormal growth among high-risk patients and to delineate factors that influence the correct classification of fetal growth.