Fractional fetal thigh volume in the prediction of normal and abnormal fetal growth during the third trimester of pregnancy

Neonatal birthweight prediction using two- and three-dimensional estimated fetal weight among borderline small fetuses

PURPOSE

This study aimed (1) to determine the degree of correlation between 2D and 3D estimated fetal weight (EFW) and neonatal birth weight (BW) among borderline small fetuses and (2) to compare the accuracy and precision of 2D and 3D EFW in BW prediction.

METHODS

A retrospective cohort study evaluated fetuses who had an ultrasound performed between January 2017 and September 2021 at a tertiary maternal center. All singleton pregnancies with 3D EFW within 4 weeks of delivery were included. Fetuses with known structural or genetic abnormalities were excluded. Pearson's correlation coefficients were determined for both 2D and 3D EFW to BW then compared using Williams' test and Fisher r to z transformation, where applicable. Mean percent difference and standard deviation were used to assess the accuracy and precision, respectively, of 2D and 3D EFWs in BW prediction.

RESULTS

Two hundred forty-eight pregnancies were included. Ultrasound studies were performed with a median interval of 2 weeks (IQR 1, 3) between ultrasound and delivery. Both 2D and 3D estimated fetal weights showed a significant correlation with birth weight (r = 0.74 and r = 0.73, respectively), indicating similar accuracy between the two techniques.

CONCLUSION

Two-dimensional and three-dimensional EFWs performed similarly in the prediction of BW in borderline small fetuses.

The NICHD Fetal 3D Study: A Pregnancy Cohort Study of Fetal Body Composition and Volumes

There's a paucity of robust normal fractional limb and organ volume standards from a large and diverse ethnic population. The Fetal 3D Study was designed to develop research and clinical applications for fetal soft tissue and organ volume assessment. The NICHD Fetal Growth Studies (2009-2013) collected 2D and 3D fetal volumes. In the Fetal 3D Study (2015-2019), sonographers performed longitudinal 2D and 3D measurements for specific fetal anatomical structures in research ultrasounds of singletons and dichorionic twins. The primary aim was to establish standards for fetal body composition and organ volumes, overall and by maternal race/ethnicity, and determine whether these standards vary for twins versus singletons. We describe the study design, methods, and details about reviewer training. Basic characteristics of this cohort, with their corresponding distributions of fetal 3D measurements by anatomical structure, are summarized. This investigation is responsive to critical data gaps in understanding serial changes in fetal subcutaneous fat, lean body mass, and organ volume in association with pregnancy complications. In the future, this cohort can answer critical questions regarding the potential influence of maternal characteristics, lifestyle factors, nutrition, and biomarker and chemical data on longitudinal measures of fetal subcutaneous fat, lean body mass, and organ volumes.

Prediction of large-for-gestational age at 36 weeks' gestation: two-dimensional ultrasound vs three-dimensional ultrasound vs magnetic resonance imaging

OBJECTIVE

To compare the performance of two-dimensional ultrasound (2D-US), three-dimensional ultrasound (3D-US) and magnetic resonance imaging (MRI) at 36 weeks' gestation in predicting the delivery of a large-for-gestational-age (LGA) neonate, defined as birth weight ≥ 95th percentile, in patients at high and low risk for macrosomia.

METHODS

This was a secondary analysis of a prospective observational study conducted between January 2017 and February 2019. Women with a singleton pregnancy at 36 weeks' gestation underwent 2D-US, 3D-US and MRI within 15 min for estimation of fetal weight. Weight estimations and birth weight were plotted on a growth curve to obtain percentiles for comparison. Participants were considered high risk if they had at least one of the following risk factors: diabetes mellitus, estimated fetal weight ≥ 90th percentile at the routine third-trimester ultrasound examination, obesity (prepregnancy body mass index ≥ 30 kg/m2) or excessive weight gain during pregnancy. The outcome was the diagnostic performance of each modality in the prediction of birth weight ≥ 95th percentile, expressed as the area under the receiver-operating-characteristics curve (AUC), sensitivity, specificity and positive and negative predictive values.

RESULTS

A total of 965 women were included, of whom 533 (55.23%) were high risk and 432 (44.77%) were low risk. In the low-risk group, the AUCs for birth weight ≥ 95th percentile were 0.982 for MRI, 0.964 for 2D-US and 0.962 for 3D-US; pairwise comparisons were non-significant. In the high-risk group, the AUCs were 0.959 for MRI, 0.909 for 2D-US and 0.894 for 3D-US. A statistically significant difference was noted between MRI and both 2D-US (P = 0.002) and 3D-US (P = 0.002), but not between 2D-US and 3D-US (P = 0.503). In the high-risk group, MRI had the highest sensitivity (65.79%) compared with 2D-US (36.84%, P = 0.002) and 3D-US (21.05%, P < 0.001), whereas 3D-US had the highest specificity (98.99%) compared with 2D-US (96.77%, P = 0.005) and MRI (96.97%, P = 0.004).

CONCLUSIONS

At 36 weeks' gestation, MRI has better performance compared with 2D-US and 3D-US in predicting birth weight ≥ 95th percentile in patients at high risk for macrosomia, whereas the performance of 2D-US and 3D-US is comparable. For low-risk patients, the three modalities perform similarly. © 2023 International Society of Ultrasound in Obstetrics and Gynecology.

Association of maternal obesity with growth of fetal fractional limb volume

BACKGROUND

Maternal obesity influences birth weight and newborn adiposity. Fetal fractional limb volume has recently been introduced as a useful parameter for the proxy of fetal adiposity. However, the association between maternal adiposity and the growth of fetal fractional limb volume has not been examined.

AIMS

To investigate the association of maternal pre-pregnancy BMI with the growth of fetal fractional limb volume.

STUDY DESIGN

Prospective cohort study.

SUBJECTS

Women with singleton uncomplicated pregnancies enrolled between July 2017 and June 2020.

OUTCOME MEASURES

Fetal fractional limb volume was assessed between 20 and 40 weeks' gestation, measured as cylindrical limb volume based on 50 % of the total diaphysis length. The measured fractional limb volume at each gestational week were converted to z-scores based on a previous report. The association between pre-pregnancy BMI and fetal fractional limb volume was examined. Maternal age, parity, gestational weight gain and fetal sex were considered as potential confounding variables.

RESULTS

Ultrasound scans of 455 fractional arm volume and thigh volume were obtained. Fractional limb volume increased linearly until the second trimester of gestation, then increased exponentially in the third trimester. Maternal pre-pregnancy BMI was significantly correlated with z-scores of fractional arm volume and thigh volume across gestation. The post-hoc analysis showed the association between pre-pregnancy BMI and fractional arm volume was significant especially between 34 and 40 weeks.

CONCLUSIONS

Maternal obesity influences the growth pattern of fetal fractional limb volume. Fractional arm volume may potentially provide a useful surrogate marker of fetal nutritional status in late gestation.

Diagnostic performance of 32 vs 36 weeks ultrasound in predicting late-onset fetal growth restriction and small-for-gestational-age neonates: a systematic review and meta-analysis

OBJECTIVE

Fetal growth restriction is an independent risk factor for fetal death and adverse neonatal outcomes. The main aim of this study was to investigate the diagnostic performance of 32 vs 36 weeks ultrasound of fetal biometry in detecting late-onset fetal growth restriction and predicting small-for-gestational-age neonates.

DATA SOURCES

A systematic search was performed to identify relevant studies published until June 2022, using the databases PubMed, Web of Science, and Scopus.

STUDY ELIGIBILITY CRITERIA

Cohort studies in low-risk or unselected singleton pregnancies with screening ultrasound performed at ≥32 weeks of gestation were used.

METHODS

The estimated fetal weight and abdominal circumference were assessed as index tests for the prediction of small for gestational age (birthweight of <10th percentile) and detecting fetal growth restriction (estimated fetal weight of <10th percentile and/or abdominal circumference of <10th percentile). The quality of the included studies was independently assessed by 2 reviewers using the Quality Assessment of Diagnostic Accuracy Studies 2 tool. For the meta-analysis, hierarchical summary area under the receiver operating characteristic curves were constructed, and quantitative data synthesis was performed using random-effects models.

RESULTS

The analysis included 25 studies encompassing 73,981 low-risk pregnancies undergoing third-trimester ultrasound assessment for growth, of which 5380 neonates (7.3%) were small for gestational age at birth. The pooled sensitivities for estimated fetal weight of <10th percentile and abdominal circumference of <10th percentile in predicting small for gestational age were 36% (95% confidence interval, 27%-46%) and 37% (95% confidence interval, 19%-60%), respectively, at 32 weeks ultrasound and 48% (95% confidence interval, 41%-56%) and 50% (95% confidence interval, 25%-74%), respectively, at 36 weeks ultrasound. The pooled specificities for estimated fetal weight of <10th percentile and abdominal circumference of <10th percentile in detecting small for gestational age were 93% (95% confidence interval, 91%-95%) and 95% (95% confidence interval, 85%-98%), respectively, at 32 weeks ultrasound and 93% (95% confidence interval, 91%-95%) and 97% (95% confidence interval, 85%-98%), respectively, at 36 weeks ultrasound. The observed diagnostic odds ratios for an estimated fetal weight of <10th percentile and an abdominal circumference of <10th percentile in detecting small for gestational age were 8.8 (95% confidence interval, 5.4-14.4) and 11.6 (95% confidence interval, 6.2-21.6), respectively, at 32 weeks ultrasound and 13.3 (95% confidence interval, 10.4-16.9) and 36.0 (95% confidence interval, 4.9-260.0), respectively, at 36 weeks ultrasound. The pooled sensitivity, specificity, and diagnostic odds ratio in predicting fetal growth restriction were 71% (95% confidence interval, 52%-85%), 90% (95% confidence interval, 79%-95%), and 25.8 (95% confidence interval, 14.5-45.8), respectively, at 32 weeks ultrasound and 48% (95% confidence interval, 41%-55%), 94% (95% confidence interval, 93%-96%), and 16.9 (95% confidence interval, 10.8-26.6), respectively, at 36 weeks ultrasound. Abdominal circumference of <10th percentile seemed to have comparable sensitivity to estimated fetal weight of <10th percentile in predicting small-for-gestational-age neonates.

CONCLUSION

An ultrasound assessment of the fetal biometry at 36 weeks of gestation seemed to have better predictive accuracy for small-for-gestational-age neonates than an ultrasound assessment at 32 weeks of gestation. However, an opposite trend was noted when the outcome was fetal growth restriction. Fetal abdominal circumference had a similar predictive accuracy to that of estimated fetal weight in detecting small-for-gestational-age neonates.

3D Fractional Limb Volume Identifies Reduced Subcutaneous and Lean Mass in Fetal Growth Restriction

OBJECTIVES

Fetal 2D and 3D fractional limb volume (FLV) measurements by ultrasound can detect fetal lean and subcutaneous mass and possibly percent body fat. Our objectives were to 1) compare FLV measurements in fetuses with fetal growth restriction (FGR) versus small for gestational age (SGA) defined by the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG)-supported international Delphi consensus and 2) correlate FLV findings with birth metrics. We hypothesize that FLV measurements will be significantly smaller in FGR versus SGA fetuses and will correlate closer with Ponderal index (PIx) in the neonate than abdominal circumference (AC).

METHODS

Patients were categorized as FGR or SGA as defined by ISUOG. Total thigh volume (TTV), volumes of lean mass (LMV), and fat mass volume (FMV) were calculated from 3D acquisitions. Measurements were compared between groups and correlated with birthweight (BW) and PIx (BW/crown-heal length).

RESULTS

The FGR group (n = 37) delivered earlier (37/2 versus 38/0; P = .0847), were lighter (2.2 kg versus 2.6 kg; P = .0003) and had lower PIx (0.023 versus 0.025; P = .0013) than SGAs (n = 22). FGRs had reduced TTV (40.6 versus 48.4 cm³ ; P = .0164), FMV (20.8 versus 25.3 cm³ ; P = .0413), and LMV (19.8 versus 23.1 cm³ ; P = .0387). AC had the highest area under the curve (0.69) for FGR. FMV was more strongly associated with PIx than the AC (P = .0032).

CONCLUSIONS

The AC and FLV measurements were significantly reduced in FGR fetuses compared to SGAs. While the AC outperformed FLV in predicting FGR, the FLV correlated best with PIx, which holds investigative promise.

Soft tissue assessment for fetal growth restriction

Contemporary clinical practice heavily relies on interpretation of population-based birth weight standards to evaluate neonatal nutrition status. Obstetricians have adopted the use of estimated fetal weight in a similar manner to estimate fetal nutritional status. However, most fetal weight prediction models overemphasize skeletal parameters such as biparietal diameter, head circumference, and femur diaphysis length. Although most EFW calculations also include abdominal circumference, this 2D growth parameter is largely defined by liver size and a small rim of subcutaneous fat. Advances in 3D ultrasound imaging and the development of more robust image analysis tools have now made it possible to reliably add a soft tissue component for fetal nutritional assessment. This chapter explains why fetal soft tissue evaluation is clinically relevant, describes different techniques for evaluating these sonographic parameters, and outlines future directions for their practical utility in the care of malnourished fetuses.

Fetal weight estimation by automated three-dimensional limb volume model in late third trimester compared to two-dimensional model: a cross-sectional prospective observational study

Background

Accurate estimation of fetal weight is important for prenatal care and for detection of fetal growth abnormalities. Prediction of fetal weight entails the indirect measurement of fetal biometry by ultrasound that is then introduced into formulae to calculate the estimated fetal weight. The aim of our study was to evaluate the accuracy of fetal weight estimation of Chinese fetuses in the third trimester using an automated three-dimensional (3D) fractional limb volume model, and to compare this model with the traditional two-dimensional (2D) model.

Methods

Prospective 2D and 3D ultrasonography were performed among women with singleton pregnancies 7 days before delivery to obtain 2D data, including fetal biparietal diameter, abdominal circumference and femur length, as well as 3D data, including the fractional arm volume (AVol) and fractional thigh volume (TVol). The fetal weight was estimated using the 2D model and the 3D fractional limb volume model respectively. Percentage error was defined as (estimated fetal weight - actual birth weight) divided by actual birth weight and multiplied by 100. Systematic errors (accuracy) were evaluated as the mean percentage error (MPE). Random errors (precision) were calculated as ±1 SD of percentage error. The intraclass correlation coefficient (ICC) was used to analyze the inter-observer reliability of the 3D ultrasound measurements of fractional limb volume.

Results

Ultrasound examination was performed on 56 fetuses at 39.6 ± 1.4 weeks’ gestation. The average birth weight of the newborns was 3393 ± 530 g. The average fetal weight estimated by the 2D model was 3478 ± 467 g, and the MPE was 3.2 ± 8.9. The average fetal weights estimated by AVol and TVol of the 3D model were 3268 ± 467 g and 3250 ± 485 g, respectively, and the MPEs were − 3.3 ± 6.6 and − 3.9 ± 6.1, respectively. For the 3D TVol model, the proportion of fetuses with estimated error ≤ 5% was significantly higher than that of the 2D model (55.4% vs. 33.9%, p < 0.05). For fetuses with a birth weight < 3500 g, the accuracy of the AVol and TVol models were better than the 2D model (− 0.8 vs. 7.0 and − 2.8 vs. 7.0, both p < 0.05). Moreover, for these fetuses, the proportions of estimated error ≤ 5% of the AVol and TVol models were 58.1 and 64.5%, respectively, significantly higher than that of the 2D model (19.4%) (both p < 0.05). The inter-observer reliability of measuring fetal AVol and TVol were high, with the ICCs of 0.921 and 0.963, respectively.

Conclusion

In this cohort, the automated 3D fractional limb volume model improves the accuracy of weight estimation in most third-trimester fetuses. Prediction accuracy of the 3D model for neonatal BW, particularly < 3500 g was higher than that of the traditional 2D model.

Predicting fetal weight by three-dimensional limb volume ultrasound (AVol/TVol) and abdominal circumference

BACKGROUND

Fetal weight is an important parameter to ensure maternal and child safety. The purpose of this study was to use three-dimensional (3D) limb volume ultrasound combined with fetal abdominal circumference (AC) measurement to establish a model to predict fetal weight and evaluate its efficiency.

METHODS

A total of 211 participants with single pregnancy (28-42 weeks) were selected between September 2017 and December 2018 in the Beijing Obstetrics and Gynecology Hospital of Capital Medical University. The upper arm (AVol)/thigh volume (TVol) of fetuses was measured by the 3D limb volume technique. Fetal AC was measured by two-dimensional ultrasound. Nine cases were excluded due to incomplete information or the interval between examination and delivery >7 days. The enrolled 202 participants were divided into a model group (134 cases, 70%) and a verification group (68 cases, 30%) by mechanical sampling method. The linear relationship between limb volume and fetal weight was evaluated using Pearson Chi-squared test. The prediction model formula was established by multivariate regression with data from the model group. Accuracy of the model formula was evaluated with verification group data and compared with traditional formulas (Hadlock, Lee2009, and INTERGROWTH-21st) by paired t-test and residual analysis. Receiver operating characteristic curves were generated to predict macrosomia.

RESULTS

AC, AVol, and TVol were linearly related to fetal weight. Pearson correlation coefficient was 0.866, 0.862, and 0.910, respectively. The prediction model based on AVol/TVol and AC was established as follows: Y = -481.965 + 12.194TVol + 15.358AVol + 67.998AC, R2adj = 0.868. The scatter plot showed that when birth weight fluctuated by 5% (i.e., 95% to 105%), the difference between the predicted fetal weight by the model and the actual weight was small. A paired t-test showed that there was no significant difference between the predicted fetal weight and the actual birth weight (t = -1.015, P = 0.314). Moreover, the residual analysis showed that the model formula's prediction efficiency was better than the traditional formulas with a mean residual of 35,360.170. The combined model of AVol/TVol and AC was superior to the Lee2009 and INTERGROWTH-21st formulas in the diagnosis of macrosomia. Its predictive sensitivity and specificity were 87.5% and 91.7%, respectively.

CONCLUSION

Fetal weight prediction model established by semi-automatic 3D limb volume combined with AC is of high accuracy, sensitivity, and specificity. The prediction model formula shows higher predictive efficiency, especially for the diagnosis of macrosomia.

TRIAL REGISTRATION

ClinicalTrials.gov, NCT03002246; https://clinicaltrials.gov/ct2/show/NCT03002246?recrs=e&cond=fetal&draw=8&rank=67.

Defining the Normal Growth Curve of Fetal Fractional Limb Volume in a Japanese Population

Fetal fractional limb volume is a useful measure for predicting birth weight and newborn adiposity; however, a normal growth curve has been reported solely in the United States. As the birth weight of neonates in Japan is significantly lower than that in the US, fetal fractional limb volume is likely to be smaller in the Japanese population. This study aimed to define the normal growth curve of fractional arm volume (AVol) and thigh volume (TVol) in the Japanese population. Ultrasound scans of 453 AVol and TVol pairs were obtained; each AVol and TVol percentile at each gestational age was calculated. The measured AVol and TVol at each gestational week were also converted to z-scores based on a previous report. The growth curves increased linearly until the second trimester and exponentially in the third trimester. Linear regression showed a significant negative correlation between gestational age and AVol and TVol z-scores. The growth pattern of fetal fractional limb volume in the Japanese population is consistent with, but smaller than, that reported in the US; this difference becomes greater as the gestational age progresses.

Tracking placental development in health and disease

Pre-eclampsia and fetal growth restriction arise from disorders of placental development and have some shared mechanistic features. Initiation is often rooted in the maldevelopment of a maternal-placental blood supply capable of providing for the growth requirements of the fetus in later pregnancy, without exerting undue stress on maternal body systems. Here, we review normal development of a placental bed with a safe and adequate blood supply and a villous placenta-blood interface from which nutrients and oxygen can be extracted for the growing fetus. We consider disease mechanisms that are intrinsic to the maternal environment, the placenta or the interaction between the two. Systemic signalling from the endocrine placenta targets the maternal endothelium and multiple organs to adjust metabolism for an optimal pregnancy and later lactation. This signalling capacity is skewed when placental damage occurs and can deliver a dangerous pathogenic stimulus. We discuss the placental secretome including glycoproteins, microRNAs and extracellular vesicles as potential biomarkers of disease. Angiomodulatory mediators, currently the only effective biomarkers, are discussed alongside non-invasive imaging approaches to the prediction of disease risk. Identifying the signs of impending pathology early enough to intervene and ameliorate disease in later pregnancy remains a complex and challenging objective.

Fetal Weight Estimation Using Automated Fractional Limb Volume With 2-Dimensional Size Parameters: A Multicenter Study

OBJECTIVES

To develop new fetal weight prediction models using automated fractional limb volume (FLV).

METHODS

A prospective multicenter study measured fetal biometry within 4 to 7 days of delivery. Three-dimensional data acquisition included the automated FLV that was based on 50% of the humerus diaphysis (fractional arm volume [AVol]) or 50% of the femur diaphysis (fractional thigh volume [TVol]) length. A regression analysis provided population sample-specific coefficients to develop 4 weight estimation models. Estimated and actual birth weights (BWs) were compared for the mean percent difference ± standard deviation of the percent differences. Systematic errors were analyzed by the Student t test, and random errors were compared by the Pitman test.

RESULTS

A total of 328 pregnancies were scanned before delivery (BW range, 825-5470 g). Only 71.3% to 72.6% of weight estimations were within 10% of actual BW using original published models by Hadlock et al (Am J Obstet Gynecol 1985; 151:333-337) and INTERGROWTH-21st (Ultrasound Obstet Gynecol 2017; 49:478-486). All predictions were accurate by using sample-specific model coefficients to minimize bias in making these comparisons (Hadlock, 0.4% ± 8.7%; INTERGROWTH-21st, 0.5% ± 10.0%; AVol, 0.3% ± 7.4%; and TVol, 0.3% ± 8.0%). Both AVol- and TVol-based models improved the percentage of correctly classified BW ±10% in 83.2% and 83.9% of cases, respectively, compared to the INTERGROWTH-21st model (73.8%; P < .01). For BW of less than 2500 g, all models slightly overestimated BW (+2.0% to +3.1%). For BW of greater than 4000 g, AVol (-2.4% ± 6.5%) and TVol (-2.3% ± 6.9%) models) had weight predictions with small systematic errors that were not different from zero (P > .05). For these larger fetuses, both AVol and TVol models correctly classified BW (±10%) in 83.3% and 87.5% of cases compared to the others (Hadlock, 79.2%; INTERGROWTH-21st, 70.8%) although these differences did not reach statistical significance.

CONCLUSIONS

In this cohort, the inclusion of automated FLV measurements with conventional 2-dimensional biometry was generally associated with improved weight predictions.

Evaluating the accuracy and precision of sonographic fetal weight estimation models in extremely early-onset fetal growth restriction

INTRODUCTION

Birthweight is a critical predictor of survival in extremely early-onset fetal growth restriction (diagnosed pre-28 weeks' gestation, with abnormal umbilical/uterine artery Doppler waveforms), therefore accurate fetal weight estimation is a crucial component of antenatal management. Currently available sonographic fetal weight estimation models were predominantly developed in populations of mixed gestational age and varying fetal weights, but not specifically tested within the context of extremely early-onset fetal growth restriction. This study aimed to determine the accuracy and precision of fetal weight estimation in this population and investigate whether model performance is affected by other factors.

MATERIAL AND METHODS

Cases where a growth scan was performed within 48 hours of delivery (n = 65) were identified from a cohort of extremely early-onset fetal growth-restricted pregnancies at a single tertiary maternity center (n = 159). Fetal biometry measurements were used to calculate estimated fetal weight using 21 previously published models. Systematic and random errors were calculated for each model and used to identify the best performing model, which in turn was used to explore the relationship between error and gestation, estimated fetal weight, fetal presentation, fetal asymmetry and amniotic fluid volume.

RESULTS

Both systematic (median 8.2%; range -44.1 to 49.5%) and random error (median 11.6%; range 9.7-23.8%) varied widely across models. The best performing model was Hadlock head circumference-abdominal circumference-femur length (HC-AC-FL), regardless of gestational age, fetal size, fetal presentation or asymmetry, with an overall systematic error of 1.5% and random error of 9.7%. Despite this, it only calculated the estimated fetal weight within 10% of birthweight in 64.6% of cases. There was a weak negative relation between mean percentage error with Hadlock HC-AC-FL and amniotic fluid volume, suggesting fetal weight is overestimated at lower liquor volumes and underestimated at higher liquor volumes (P = 0.002, adjusted R²  = 0.08).

CONCLUSIONS

Hadlock HC-AC-FL is the most accurate model currently available to estimate fetal weight in extremely early-onset fetal growth restriction independent of gestation or fetal size, asymmetry or presentation. However, for 35.4% of cases in this study, estimated fetal weight calculated using this model deviates by more than 10% from birthweight, highlighting a need for an improved model.

ISUOG Practice Guidelines: ultrasound assessment of fetal biometry and growth

INTRODUCTION These Guidelines aim to describe appropriate assessment of fetal biometry and diagnosis of fetal growth disorders. These disorders consist mainly of fetal growth restriction (FGR), also referred to as intrauterine growth restriction (IUGR) and often associated with small‐for‐gestational age (SGA), and large‐for‐gestational age (LGA), which may lead to fetal macrosomia; both have been associated with a variety of adverse maternal and perinatal outcomes. Screening for, and adequate management of, fetal growth abnormalities are essential components of antenatal care, and fetal ultrasound plays a key role in assessment of these conditions. The fetal biometric parameters measured most commonly are biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC) and femur diaphysis length (FL). These biometric measurements can be used to estimate fetal weight (EFW) using various different formulae1. It is important to differentiate between the concept of fetal size at a given timepoint and fetal growth, the latter being a dynamic process, the assessment of which requires at least two ultrasound scans separated in time. Maternal history and symptoms, amniotic fluid assessment and Doppler velocimetry can provide additional information that may be used to identify fetuses at risk of adverse pregnancy outcome. Accurate estimation of gestational age is a prerequisite for determining whether fetal size is appropriate‐for‐gestational age (AGA). Except for pregnancies arising from assisted reproductive technology, the date of conception cannot be determined precisely. Clinically, most pregnancies are dated by the last menstrual period, though this may sometimes be uncertain or unreliable. Therefore, dating pregnancies by early ultrasound examination at 8–14 weeks, based on measurement of the fetal crown–rump length (CRL), appears to be the most reliable method to establish gestational age. Once the CRL exceeds 84 mm, HC should be used for pregnancy dating2–4. HC, with or without FL, can be used for estimation of gestational age from the mid‐trimester if a first‐trimester scan is not available and the menstrual history is unreliable. When the expected delivery date has been established by an accurate early scan, subsequent scans should not be used to recalculate the gestational age1. Serial scans can be used to determine if interval growth has been normal. In these Guidelines, we assume that the gestational age is known and has been determined as described above, the pregnancy is singleton and the fetal anatomy is normal. Details of the grades of recommendation used in these Guidelines are given in Appendix 1. Reporting of levels of evidence is not applicable to these Guidelines.

Diagnostic performance of third-trimester ultrasound for the prediction of late-onset fetal growth restriction: a systematic review and meta-analysis

OBJECTIVE

The objective of the study was to establish the diagnostic performance of ultrasound screening for predicting late smallness for gestational age and/or fetal growth restriction.

DATA SOURCES

A systematic search was performed to identify relevant studies published since 2007 in English, Spanish, French, Italian, or German, using the databases PubMed, ISI Web of Science, and SCOPUS.

STUDY ELIGIBILITY CRITERIA

We used rrospective and retrospective cohort studies in low-risk or nonselected singleton pregnancies with screening ultrasound performed at ≥32 weeks of gestation.

STUDY APPRAISAL AND SYNTHESIS METHODS

The estimated fetal weight and fetal abdominal circumference were assessed as index tests for the prediction of birthweight <10th (i.e. smallness for gestational age), less than the fifth, and less than the third centile and fetal growth restriction (estimated fetal weight less than the third or estimated fetal weight <10th plus Doppler signs). Quality of the included studies was independently assessed by 2 reviewers, using the Quality Assessment of Diagnostic Accuracy Studies-2 tool. For the meta-analysis, hierarchical summary receiver-operating characteristic curves were constructed, and quantitative data synthesis was performed using random-effects models. The sensitivity of the abdominal circumference <10th centile and estimated fetal weight <10th centile for a fixed 10% false-positive rate was derived from the corresponding hierarchical summary receiver-operating characteristic curves. Heterogeneity between studies was visually assessed using Galbraith plots, and publication bias was assessed by funnel plots and quantified by Deeks' method.

RESULTS

A total of 21 studies were included. Observed pooled sensitivities of abdominal circumference and estimated fetal weight <10th centile for birthweight <10th centile were 35% (95% confidence interval, 20-52%) and 38% (95% confidence interval, 31-46%), respectively. Observed pooled specificities were 97% (95% confidence interval, 95-98%) and 95% (95% confidence interval, 93-97%), respectively. Modeled sensitivities of abdominal circumference and estimated fetal weight <10th centile for 10% false-positive rate were 78% (95% confidence interval, 61-95%) and 54% (95% confidence interval, 46-52%), respectively. The sensitivity of estimated fetal weight <10th centile was better when aimed to fetal growth restriction than to smallness for gestational age. Meta-regression analysis showed a significant increase in sensitivity when ultrasound evaluation was performed later in pregnancy (P = .001).

CONCLUSION

Third-trimester abdominal circumference and estimated fetal weight perform similar in predicting smallness for gestational age. However, for a fixed 10% false-positive rate extrapolated sensitivity is higher for abdominal circumference. There is evidence of better performance when the scan is performed near term and when fetal growth restriction is the targeted condition.

Two-dimensional fetal biometry versus three-dimensional fractional thigh volume for ultrasonographic prediction of birthweight

OBJECTIVE

To develop and validate birthweight prediction models using fetal fractional thigh volume (TVol) in an Indian population, comparing them with existing prediction models developed for other ethnicities.

METHODS

A prospective observational study was conducted among 131 pregnant women (>36 weeks) attending a tertiary hospital in New Delhi, India, for prenatal care between December 1, 2014, and November 1, 2016. Participants were randomly divided into formulating (n=100) and validation (n=31) groups. Multiple regression analysis was performed to generate four models to predict birthweight using various combinations of two-dimensional (2D) ultrasonographic parameters and a three-dimensional (3D) ultrasonographic parameter (TVol). The best fit model was compared with previously published 2D and 3D models.

RESULTS

The best fit model comprised biparietal diameter, head circumference, abdominal circumference, and TVol. This model had the lowest mean percentage error (0.624 ± 8.075) and the highest coefficient of determination (R² =0.660). It correctly predicted 70.2% and 91.6% of birthweights within 5% and 10% of actual weight, respectively. Compared with previous models, attributability for the 2D and 3D models was 0.65 and 0.55, respectively. Accuracy was -0.05 ± 1.007 and -2.54 ± 1.11, respectively.

CONCLUSION

Models that included TVol provided good prediction of birthweight in the target population.

Diagnosis and surveillance of late-onset fetal growth restriction

By consensus, late fetal growth restriction is that diagnosed >32 weeks. This condition is mildly associated with a higher risk of perinatal hypoxic events and suboptimal neurodevelopment. Histologically, it is characterized by the presence of uteroplacental vascular lesions (especially infarcts), although the incidence of such lesions is lower than in preterm fetal growth restriction. Screening procedures for fetal growth restriction need to identify small babies and then differentiate between those who are healthy and those who are pathologically small. First- or second-trimester screening strategies provide detection rates for late smallness for gestational age <50% for 10% of false positives. Compared to clinically indicated ultrasonography in the third trimester, universal screening triples the detection rate of late smallness for gestational age. As opposed to early third-trimester ultrasound, scanning late in pregnancy (around 37 weeks) increases the detection rate for birthweight <3rd centile. Contrary to early fetal growth restriction, umbilical artery Doppler velocimetry alone does not provide good differentiation between late smallness for gestational age and fetal growth restriction. A combination of biometric parameters (with severe smallness usually defined as estimated fetal weight or abdominal circumference <3rd centile) with Doppler criteria of placental insufficiency (either in the maternal [uterine Doppler] or fetal [cerebroplacental ratio] compartments) offers a classification tool that correlates with the risk for adverse perinatal outcome. There is no evidence that induction of late fetal growth restriction at term improves perinatal outcomes nor is it a cost-effective strategy, and it may increase neonatal admission when performed <38 weeks.

A new customized fetal growth standard for African American women: the PRB/NICHD Detroit study

BACKGROUND

The assessment of fetal growth disorders requires a standard. Current nomograms for the assessment of fetal growth in African American women have been derived either from neonatal (rather than fetal) biometry data or have not been customized for maternal ethnicity, weight, height, and parity and fetal sex.

OBJECTIVE

We sought to (1) develop a new customized fetal growth standard for African American mothers; and (2) compare such a standard to 3 existing standards for the classification of fetuses as small (SGA) or large (LGA) for gestational age.

STUDY DESIGN

A retrospective cohort study included 4183 women (4001 African American and 182 Caucasian) from the Detroit metropolitan area who underwent ultrasound examinations between 14-40 weeks of gestation (the median number of scans per pregnancy was 5, interquartile range 3-7) and for whom relevant covariate data were available. Longitudinal quantile regression was used to build models defining the "normal" estimated fetal weight (EFW) centiles for gestational age in African American women, adjusted for maternal height, weight, and parity and fetal sex, and excluding pathologic factors with a significant effect on fetal weight. The resulting Perinatology Research Branch/Eunice Kennedy Shriver National Institute of Child Health and Human Development (hereinafter, PRB/NICHD) growth standard was compared to 3 other existing standards--the customized gestation-related optimal weight (GROW) standard; the Eunice Kennedy Shriver National Institute of Child Health and Human Development (hereinafter, NICHD) African American standard; and the multinational World Health Organization (WHO) standard--utilized to screen fetuses for SGA (<10th centile) or LGA (>90th centile) based on the last available ultrasound examination for each pregnancy.

RESULTS

First, the mean birthweight at 40 weeks was 133 g higher for neonates born to Caucasian than to African American mothers and 150 g higher for male than female neonates; maternal weight, height, and parity had a positive effect on birthweight. Second, analysis of longitudinal EFW revealed the following features of fetal growth: (1) all weight centiles were about 2% higher for male than for female fetuses; (2) maternal height had a positive effect on EFW, with larger fetuses being affected more (2% increase in the 95th centile of weight for each 10-cm increase in height); and (3) maternal weight and parity had a positive effect on EFW that increased with gestation and varied among the weight centiles. Third, the screen-positive rate for SGA was 7.2% for the NICHD African American standard, 12.3% for the GROW standard, 13% for the WHO standard customized by fetal sex, and 14.4% for the PRB/NICHD customized standard. For all standards, the screen-positive rate for SGA was at least 2-fold higher among fetuses delivered preterm than at term. Fourth, the screen-positive rate for LGA was 8.7% for the GROW standard, 9.2% for the PRB/NICHD customized standard, 10.8% for the WHO standard customized by fetal sex, and 12.3% for the NICHD African American standard. Finally, the highest overall agreement among standards was between the GROW and PRB/NICHD customized standards (Cohen's interrater agreement, kappa = 0.85).

CONCLUSION

We developed a novel customized PRB/NICHD fetal growth standard from fetal data in an African American population without assuming proportionality of the effects of covariates, and without assuming that these effects are equal on all centiles of weight; we also provide an easy-to-use centile calculator. This standard classified more fetuses as being at risk for SGA compared to existing standards, especially among fetuses delivered preterm, but classified about the same number of LGA. The comparison among the 4 growth standards also revealed that the most important factor determining agreement among standards is whether they account for the same factors known to affect fetal growth.

Individualized growth assessment: conceptual framework and practical implementation for the evaluation of fetal growth and neonatal growth outcome

Fetal growth abnormalities can pose significant consequences on perinatal morbidity and mortality of nonanomalous fetuses. The most widely accepted definition of fetal growth restriction is an estimated fetal weight less than the 10th percentile for gestational age according to population-based criteria. However, these criteria do not account for the growth potential of an individual fetus, nor do they effectively separate constitutionally small fetuses from ones that are malnourished. Furthermore, conventional approaches typically evaluate estimated fetal weight at a single time point, rather than using serial scans, to evaluate growth. This article provides a conceptual framework for the individualized growth assessment of a fetus/neonate based on measuring second-trimester growth velocity of fetal size parameters to estimate growth potential. These estimates specify size models that generate individualized third-trimester size trajectories and predict birth characteristics. Comparisons of measured and predicted values are used to separate normally growing fetuses from those with growth abnormalities. This can be accomplished with individual anatomical parameters or sets of parameters. A practical and freely available software (Individualized Growth Assessment Program) has been developed to allow implementation of this approach for clinical and research purposes.

Comparison of conventional 2D ultrasound to magnetic resonance imaging for prenatal estimation of birthweight in twin pregnancy

BACKGROUND

During prenatal follow-up of twin pregnancies, accurate identification of birthweight and birthweight discordance is important to identify the high-risk group and plan perinatal care. Unfortunately, prenatal evaluation of birthweight discordance by 2-dimensional ultrasound has been far from optimal.

OBJECTIVE

The objective of the study was to prospectively compare estimates of fetal weight based on 2-dimensional ultrasound (ultrasound-estimated fetal weight) and magnetic resonance imaging (magnetic resonance-estimated fetal weight) with actual birthweight in women carrying twin pregnancies.

STUDY DESIGN

Written informed consent was obtained for this ethics committee-approved study. Between September 2011 and December 2015 and within 48 hours before delivery, ultrasound-estimated fetal weight and magnetic resonance-estimated fetal weight were conducted in 66 fetuses deriving from twin pregnancies at 34.3-39.0 weeks; gestation. Magnetic resonance-estimated fetal weight derived from manual measurement of fetal body volume. Comparison of magnetic resonance-estimated fetal weight and ultrasound-estimated fetal weight measurements vs birthweight was performed by calculating parameters as described by Bland and Altman. Receiver-operating characteristic curves were constructed for the prediction of small-for-gestational-age neonates using magnetic resonance-estimated fetal weight and ultrasound-estimated fetal weight. For twins 1 and 2 separately, the relative error or percentage error was calculated as follows: (birthweight - ultrasound-estimated fetal weight (or magnetic resonance-estimated fetal weight)/birthweight) × 100 (percentage). Furthermore, ultrasound-estimated fetal weight, magnetic resonance-estimated fetal weight, and birthweight discordance were calculated as 100 × (larger estimated fetal weight-smaller estimated fetal weight)/larger estimated fetal weight. The ultrasound-estimated fetal weight discordance and the birthweight discordance were correlated using linear regression analysis and Pearson's correlation coefficient. The same was done between the magnetic resonance-estimated fetal weight and birthweight discordance. To compare data, the χ², McNemar test, Student t test, and Wilcoxon signed rank test were used as appropriate. We used the Fisher r-to-z transformation to compare correlation coefficients.

RESULTS

The bias and the 95% limits of agreement of ultrasound-estimated fetal weight are 2.99 (-19.17% to 25.15%) and magnetic resonance-estimated fetal weight 0.63 (-9.41% to 10.67%). Limits of agreement were better between magnetic resonance-estimated fetal weight and actual birthweight as compared with the ultrasound-estimated fetal weight. Of the 66 newborns, 27 (40.9%) were of weight of the 10th centile or less and 21 (31.8%) of the fifth centile or less. The area under the receiver-operating characteristic curve for prediction of birthweight the 10th centile or less by prenatal ultrasound was 0.895 (P < .001; SE, 0.049), and by magnetic resonance imaging it was 0.946 (P < .001; SE, 0.024). Pairwise comparison of receiver-operating characteristic curves showed a significant difference between the areas under the receiver-operating characteristic curves (difference, 0.087, P = .049; SE, 0.044). The relative error for ultrasound-estimated fetal weight was 6.8% and by magnetic resonance-estimated fetal weight, 3.2% (P < .001). When using ultrasound-estimated fetal weight, 37.9% of fetuses (25 of 66) were estimated outside the range of ±10% of the actual birthweight, whereas this dropped to 6.1% (4 of 66) with magnetic resonance-estimated fetal weight (P < .001). The ultrasound-estimated fetal weight discordance and the birthweight discordance correlated significantly following the linear equation: ultrasound-estimated fetal weight discordance = 0.03 + 0.91 × birthweight (r = 0.75; P < .001); however, the correlation was better with magnetic resonance imaging: magnetic resonance-estimated fetal weight discordance = 0.02 + 0.81 × birthweight (r = 0.87; P < .001).

CONCLUSION

In twin pregnancies, magnetic resonance-estimated fetal weight performed immediately prior to delivery is more accurate and predicts small-for-gestational-age neonates significantly better than ultrasound-estimated fetal weight. Prediction of birthweight discordance is better with magnetic resonance imaging as compared with ultrasound.