Sonographic estimates of fetal weight in the intrauterine growth retardation population

Comparing INTERGROWTH-21st Century and Hadlock growth standards to predict small for gestational age and short-term neonatal outcomes

Objective: To compare the INTERGROWTH-21st Century growth standard to the Hadlock standard in predicting small for gestational age (SGA) and adverse neonatal outcomes.Method: This is a prospective cohort study on women with singleton gestations referred for fetal growth ultrasound between 26.0 and 36.6 weeks gestational age (GA). The primary outcome is prediction of neonatal SGA. Neonatal SGA was defined as birthweight <10th percentile for GA by Alexander chart. The discriminatory ability of the growth standards was compared using area under receiver operating characteristic curves (AUC).Results: Among 1054 patients who met inclusion criteria, 139 (13.2%) had neonatal SGA. The mean interval between estimated fetal weight and birthweight was 6.7 ± 3.1 weeks. Composite adverse neonatal outcome was seen in 300 (28.4%) patients. The sensitivity for identifying SGA neonates was higher for Hadlock compared with INTERGROWTH-21st standard (41.7 vs. 24.5%); AUC (95% CI) were 0.69 (0.65-0.73) and 0.62 (0.58-0.65), respectively. Both standards were comparable in predicting the composite adverse neonatal outcomes; AUC (95% CI) were 0.52 (0.50-0.53) and 0.52 (0.50-0.54), respectively; p = .28.Conclusions: The Hadlock standard had a moderate but higher discriminatory ability for predicting neonatal SGA compared to the INTERGROWTH-21st project standard. However, the two standards were poor predictors of early adverse neonatal outcomes.Rationale: The Intergrowth-21st project was recently introduced with the goal of providing a universal benchmark for comparing growth across different ethnicity. We performed a prospective cohort study to compare the Intergrowth-21st standard with the commonly used Hadlock standard for predicting pregnancies at risk for neonatal SGA and adverse outcomes. Hadlock fetal growth standard is moderately superior at predicting neonatal SGA compared to the Intergrowth-21st standard. Both standards are poor at predicting adverse neonatal outcomes. These findings, however, need further validation.

Fetal growth: a review of terms, concepts and issues relevant to obstetrics

The perinatal literature includes several potentially confusing and controversial terms and concepts related to fetal size and growth. This article discusses fetal growth from an obstetric perspective and addresses various issues including the physiologic mechanisms that determine fetal growth trajectories, known risk factors for abnormal fetal growth, diagnostic and prognostic issues related to restricted and excessive growth and temporal trends in fetal growth. Also addressed are distinctions between fetal growth 'standards' and fetal growth 'references', and between fetal growth charts based on estimated fetal weight vs those based on birth weight. Other concepts discussed include the incidence of fetal growth restriction in pregnancy (does the frequency of fetal growth restriction increase or decrease with increasing gestation?), the obstetric implications of studies showing associations between fetal growth and adult chronic illnesses (such as coronary heart disease) and the need for customizing fetal growth standards.

Which is the most accurate formula to estimate fetal weight in women with severe preterm preeclampsia?

OBJECTIVE

To identify the most accurate formula to estimate fetal weight (EFW) from ultrasound parameters in severe preterm preeclampsia.

METHODS

In a prospective study, serial ultrasound assessments were performed in 123 women with severe preterm preeclampsia. The EFW, calculated for 111 live born, normal, singleton fetuses within 7 days of delivery using 38 published formulae, was compared to the actual birth weight (ABW). Accuracy was assessed by correlations, mean (absolute and signed) (%) errors, % correct predictions within 5-20% of ABW and limits of agreement.

RESULTS

Accuracy was highly variable. Most formulae systematically overestimated ABW. Five Hadlock formulae utilizing three or four variables and Woo 3 formula had the highest accuracy and did not differ significantly (mean absolute % errors 6.8-7.2%, SDs 5.3-5.8%, > 75% of estimations within 10% of ABW and 95% limits of agreement between -18/20% and +14/15%). They were not negatively affected by clinical variables but had some inconsistency in bias over the ABW range. All other formulae, including those targeted for small, preterm or growth restricted fetuses, were inferior and/or affected by multiple clinical variables.

CONCLUSION

In this GA window, Hadlock formulae using three or four variables or Woo 3 formula can be recommended.

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

PURPOSE

To determine the factors that might influence the accuracy of sonographic estimated fetal weight.

STUDY DESIGN

A PubMed search (Jan 1975 to Jan 2003) of articles published in the English language was carried out and the inclusion criterion was that estimates were within 10% of birth weight. A Chi-square test for trend was used and odds ratio (OR) with 95% confidence intervals (CI) was calculated.

RESULTS

Over 28 years, 175 articles were identified but only 54 (31%) met the inclusion criterion. Overall 62% (8895/14 384) of the predictions were within 10% of the actual weight. The accuracy was significantly different in articles where <7 vs. >7 days were allowed to lapse between examination and delivery (OR 2.17, 95% CI 1.93, 2.45); where examinations were done by registered diagnostic medical sonographers (RDMS; 65%) versus physicians (59%) or residents (57%; p < 0.0001); in term vs. preterm patients (OR 1.97, 95% CI 1.67, 2.13); and in studies with >1000 vs. <1000 cohorts (OR 1.62; 95% CI 1.51, 1.74).

CONCLUSIONS

If feasible the sonographic examination should be done by RDMS and within a week of delivery.

Accuracy of a single fetal weight estimation at 29-34 weeks in diabetic pregnancies: can it predict large-for-gestational-age infants at term?

OBJECTIVES

This study was undertaken to evaluate the accuracy of a single sonographic estimated fetal weight at 29-34 weeks' gestation with respect to birthweight determination in diabetic pregnancies.

STUDY DESIGN

A retrospective cohort study of 423 diabetic pregnancies with detailed fetal measurements at 29-34 weeks' gestation. Multivariate regression analysis was used to predict the birthweight. The percentiles of the estimated fetal weight and the calculated birthweight were compared with the actual birthweight percentile.

RESULTS

The mean birthweight percentile at term was significantly higher than the estimated fetal weight percentile at 29-34 weeks' gestation in the women with poor glycemic control, but not the women with good control. On multivariate analysis, the estimated fetal weight, interval from ultrasound to delivery, hemoglobin A1C level, gestational age at ultrasound, and classification of glycemic control were independently associated with the birthweight. Both the estimated fetal weight and the calculated birthweight had a low sensitivity and a low positive predictive value for predicting large-for-gestational-age infants.

CONCLUSION

Accelerated fetal growth is evident primarily in diabetic women with poor glycemic control. These fetuses cannot be identified by a single ultrasound examination at 29-34 weeks' gestation.

Predictive value of a single early fetal weight estimate in normal pregnancies

OBJECTIVES

To evaluate the accuracy of ultrasound-based fetal weight estimates made at 28-34 weeks of gestation in predicting small- and large-for-gestational-age infants (SGA, LGA) at term.

METHODS

Two-hundred and fifty-nine patients with a healthy, singleton pregnancy in whom fetal biometry measurements were routinely performed between 28 and 34 weeks' gestation, were recruited at term delivery. The sonographic estimated fetal weight (EFW) and the birth weight were converted to percentiles on the basis of locally developed growth charts and compared. Multivariate linear stepwise regression analysis was used to predict the birth weight and birth weight percentile. The resulting equation (projectile formula) was used to determine the calculated birth weight, and that value was compared with the actual birth weight. The Bland and Altman plot and Passing and Bablok regression were used to compare between the calculated birth weight and the actual birth weight.

RESULTS

Mean gestational age at ultrasound examination was 32+/-1.6 weeks (28-34), and mean age at delivery was 39+/-1.7 weeks (37-42). The multivariate correlation between the calculated birth weight and the birth weight (R2 = 0.524) was higher than the correlation between the sonographic EFW and the birth weight (R2 = 0.083). Both the sonographic EFW and the calculated birth weight are characterized by low positive predictive values in predicting SGA or LGA infants. The calculated birth weight was more accurate in excluding SGA and LGA infants (negative predictive values of 99.5% and 100%, respectively). On method comparison tests, the calculated birth weight was not significantly different than the actual birth weight.

CONCLUSIONS

Fetal weight estimation at the early third trimester poorly predicts the birth weight centile at term. It remains uncertain, however, if it would be useful to use the calculated birth weight in pregnancies with clinically suspected SGA or LGA fetuses.

Accuracy of sonographically estimated fetal weight in 840 women with different pregnancy complications prior to induction of labor

OBJECTIVES

To evaluate the accuracy of sonographically estimated fetal weight (EFW) shortly before induction of labor in the presence of different pregnancy complications, and to define possible variables affecting it.

METHODS

The study sample consisted of 840 women with singleton pregnancies and cephalic presentation who were admitted to our unit for induction of labor between January 1999 and December 2000. All underwent detailed ultrasound assessment for EFW, amniotic fluid index, biophysical profile and placental location. Indications included previous Cesarean section, postdate pregnancy, pregnancy-induced hypertension, diabetic pregnancy, suspected large-for-gestational age (LGA) infants, suspected fetal growth restriction (FGR), oligohydramnios, decreased fetal movements, premature rupture of membranes at or before term. EFW was calculated after measuring fetal abdominal circumference and femur length. The EFW was compared with the weight at delivery, 1-3 days later.

RESULTS

There was a high correlation between EFW and birth weight (R(2) = 0.775, P < 0.001). The mean birth weight was 3207 +/- 561 g, and mean absolute weight difference was 227 +/- 197 g; (absolute range, 0-1700 g; actual range, - 986 to + 1700 g). The mean weight difference was significantly different between the patients with LGA infants, FGR infants and preterm premature rupture of membranes (PPROM) (- 110 +/- 281 g, + 113 +/- 195 g and + 115 +/- 307 g, respectively, P < 0.01). Stepwise linear regression analysis of the effects of maternal and pregnancy characteristics on the weight difference yielded lower gestational age, higher birth weight, anterior placenta, higher gravidity, and younger maternal age as independent and significant variables associated with greater actual weight difference inaccuracy (R(2) = 0.099, P < 0.001), and higher birth weight as the only independent and significant variable associated with greater absolute weight difference (R(2) = 0.09, P = 0.018).

CONCLUSIONS

The sonographic EFW is highly correlated with birth weight. However, clinicians should be aware of the risk of overestimation in pregnancies with suspected LGA and underestimation in pregnancies with PPROM and suspected FGR.

Ultrasonic fetal weight estimation and tolerance to measurement error: A comparative analysis

The objective of the present study was to test the hypothesis that volumetric formulae for fetal weight estimation are more tolerant to measurement error than exponential models. A mathematical model of normal fetal growth was constructed, using published British reference standards for biometric variables. Observed measurements were computer-generated by contaminating reference measurements with error terms according to their published coefficients of variation. The error in weight estimation was computed as the percentage difference between weight estimates derived from the observed biometric variables and the true measurements. A total of nine weight estimation formulae were tested. Campbell's formula appeared to be most affected by observational errors, especially before 38 weeks. In this range, they varied up to 9%. The most tolerant was Shepard's formula, with errors of only approximately 2.8%. Other formulae showed errors of approximately 5-6%. With the exception of Campbell's formula, the effect of gestational age was minimal. There was no correlation between percentage error and fetal size. Combining ultrasound biometric variables into a fetal weight estimate does not always exaggerate the errors of the original measurements. There were no significant differences between volumetric and exponential formulae.

Menstrual age-dependent systematic error in sonographic fetal weight estimation: a mathematical model

PURPOSE

We used computer modeling techniques to evaluate the accuracy of different types of sonographic formulas for estimating fetal weight across the full range of clinically important menstrual ages.

METHODS

Input data for the computer modeling techniques were derived from published British standards for normal distributions of sonographic biometric growth parameters and their correlation coefficients; these standards had been derived from fetal populations whose ages were determined using sonography. The accuracy of each of 10 formulas for estimating fetal weight was calculated by comparing the weight estimates obtained with these formulas in simulated populations with the weight estimates expected from birth weight data, from 24 weeks' menstrual age to term. Preterm weights were estimated by interpolation from term birth weights using sonographic growth curves. With an ideal formula, the median weight estimates at term should not differ from the population birth weight median.

RESULTS

The simulated output sonographic values closely matched those of the original population. The accuracy of the fetal weight estimation differed by menstrual age and between various formulas. Most methods tended to overestimate fetal weight at term. Shepard's formula progressively overestimated weights from about 2% at 32 weeks to more than 15% at term. The accuracy of Combs's and Shinozuka's volumetric formulas varied least by menstrual age. Hadlock's formula underestimated preterm fetal weight by up to 7% and overestimated fetal weight at term by up to 5%.

CONCLUSIONS

The accuracy of sonographic fetal weight estimation based on volumetric formulas is more consistent across menstrual ages than are other methods.

Fetal biometry: clinical, pathological, and technical considerations

Sonographic measurements of fetal ultrasound parameters are the basis for accurate determination of gestational age and detection of fetal growth abnormalities. Selection of the most useful single biometric parameter depends on the timing and purpose of measurement and is influenced by specific limitations. CRL (crown-rump length) is the best parameter for early dating of pregnancy. Biparietal diameter (BPD) maintains the closest correlation with gestational age in the second trimester. In cases of variation in the shape of the skull, head circumference is an effective alternative. Abdominal circumference is the most useful dimension to evaluate fetal growth, and femur length is the best parameter in the evaluation of skeletal dysplasia. Use of multiple predictors improves the accuracy of estimates. An individual approach to each pregnancy is recommended for fetal growth assessment. The various epidemiological factors involved in fetal growth should be considered and specific charts for different communities should be used when possible. The methods of fetal weight estimation with their limitations and potential errors are presented. Clinical application of fetal biometry in abnormal growth is discussed in cases of small- and large-for-gestational-age fetuses, chromosomal aberrations, and skeletal dysplasias.

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.

Prediction of light-for-gestational age at delivery in twin pregnancies: an evaluation of fetal weight deviation and growth discordance measured by ultrasound

The purpose of this retrospective study was to evaluate and discuss different ultrasound methods widely used, among other things, as predictors for light-for-gestational age (LGA) in twin pregnancies. The methods evaluated and compared as predictors for LGA at birth were: (1) Difference between twins in biparietal diameter; (2) difference in abdominal diameter; (3) the percentage difference in estimated fetal weight between twins; and (4) estimation of the weight deviation from the expected weight during pregnancy. The study comprised 66 twin pregnancies, examined by ultrasound scanning less than 15 days before delivery. Using Relative Operating Characteristic curves (ROC curves) estimated fetal weight deviation was the most sensitive and specific of the methods. It is stressed that fetal discordance is not the appropriate predictor of LGA at birth in twin pregnancies.