How Accurate Is a Doctor Guessing How Much Baby in Womb Weighs

Ultrasound. 2018 February; 26(1): 32–41.

The accuracy of ultrasound interpretation of fetal weight in comparison to birth weight: A systematic review

Julia Milner

^aneRotherham NHS Foundation Trust, Rotherham, UK

Jane Arezina

²Academy of Leeds, Leeds, UK

Received 2017 Apr 26; Accepted 2017 Aug 20.

Abstract

Ultrasound estimation of fetal weight is a highly influential factor in antenatal management, guiding both the timing and mode of commitment of a pregnancy. Although substantial enquiry has investigated the most accurate ultrasound formula for computing estimated fetal weight, electric current bear witness indicates significant fault levels. The aim of this systematic review was to identify the most accurate method, whilst identifying sources of inaccuracy in order to facilitate recommendations for future practice. 7 studies met the inclusion criteria and 11 different formulae were assessed; ultrasound calculation of fetal weight was most commonly overestimated. The Hadlock A formula produced the most accurate results, with the everyman levels of random error. Methods incorporating only ii measurement parameters were inconsistent, producing large random errors across multiple studies. Key sources of inaccuracy included difficulties obtaining authentic fetal measurements in tardily gestation; the residual were operator dependent, including lack of experience and insufficient training and audit. The accuracy of ultrasound estimated fetal weight has improved in the last decade, though a lack of consistency remains evident. National implementation of a rigorous audit program would likely improve accuracy further, and increase the confidence and clinical value of the method.

Keywords: Ultrasound, estimated fetal weight, birth weight, accuracy

Introduction

The assessment of fetal growth is a critical component of prenatal intendance, enabling identification of fetuses at risk of perinatal morbidities or mortality.^ane Recognition of both fetal growth restriction (FGR) and big for gestational age (LGA) fetuses is essential to programme appropriate care.ⁱⁱ FGR, referring to fetuses with a birth weight plotting below the 10th percentile, is the single strongest chance factor for stillbirth.³ LGA fetuses, those with a birth weight greater than the 90th percentile,ⁱⁱ are at take a chance of shoulder dystocia and thus increased emergency caesarean section rates.²

Following the introduction of revised FGR management guidelines^one to reduce stillbirth rates,^four ultrasound evaluation of fetal growth has become more than widely performed.¹ Amongst other assessments, an ultrasound growth scan incorporates the operation of three fetal biometry measurements – caput circumference (HC), abdominal circumference (Air conditioning) and femur length (FL).^five Specific standards and landmarks required for each measurement are stipulated by the National Wellness Service Fetal Anomaly Screening Programme,⁶ to ensure accuracy and reproducibility and reduce both inter- and intra-operator variability. The biparietal diameter (BPD) measurement of the fetal head was previously performed in preference to the HC, though this practice is at present considered outdated in the United kingdom of great britain and northern ireland, in accord with the British Medical Ultrasound Order (BMUS).^v

Whilst historically the 3 fetal biometry measurements (HC, Air conditioning, FL) taken during the scan were plotted on individual population-based charts,^{5,seven – 9} the measurements are now combined to calculate an estimated fetal weight (EFW),¹ plotted on a customised growth chart (CGC).^x The CGC was offset introduced into practice over a decade ago, though has become widely utilised during the last 5 years.^ten CGCs are personalised to incorporate maternal constitutional characteristics, including body mass index (BMI) and ethnicity, to predict the optimal fetal growth bend for an individual pregnancy.¹⁰ Consequently, any deviation in growth is likely to represent pathology, rather than physiological variation.¹⁰

Substantial research has investigated the most accurate formula for calculating EFW.^{xi – xiii} During recent years, formulae adapted to specific populations, for example twin pregnancies or macrosomic fetuses, and those incorporating iii-dimensional (3D) ultrasound imaging take been evaluated, though they are yet to become clinically established.¹³ As such, in electric current U.k. clinical exercise, the RCOG¹ and BMUS advocate the apply of the Hadlock A formula¹⁴; a regression method combining all 3 measurements.⁵ Although most evidence supports formulae incorporating all three parameters,^{15 – 17} the Air conditioning measurement is the near sensitive private indicator of aberrant fetal growth.^18,xix

In 2005, a systematic review assessing the accuracy of ultrasound EFW constitute the Hadlock A formula^xiv produced the smallest systematic hateful errors on a normal fetal population.^eleven Whilst this evidence was reinforced more recently,¹² large random error levels persist.^16,20 Most significantly, calculation of EFW using ultrasound is generally overestimated, peculiarly in the population of modest fetuses,^12,20 raising concerns regarding increasing levels of obstetric intervention.^21,22 When because the direction of delivery, EFW should be authentic to within 5%, though 10% is considered adequate,^eleven,23 thus minimising random error levels is disquisitional.^eleven,15

The aim of this review was to assess the present accuracy of ultrasound calculation of EFW, identifying the most consistent formula, whilst establishing the key factors currently affecting accuracy.

Method

Report selection criteria

Pre-specified inclusion and exclusion criteria were utilised. Studies published from 2005 onwards were sought that compared the ultrasound calculation of EFW to the bodily birth weight of the neonate, within a vii-24-hour interval scan to delivery interval, to appraise the degree of accuracy. All studies incorporated data from 24 to 42 weeks gestational age at delivery. Written report pattern was not restricted and there was no stipulation of sampling method; both selected and random groups of participants were included. Multi-ethnic studies, performed both inside and exterior of the UK, were incorporated to heighten the strength and generalisability of the results,^11,xviii providing the method had not been adjusted to a specific ethnic population. Studies were excluded that incorporated data from multiple pregnancies and known fetal anomalies, and besides those focusing exclusively on specialist equations for FGR and macrosomia.²⁴ There were no restrictions regarding maternal characteristics, including BMI, to ensure the review was representative of a typical population.

Studies were only included if the national screening standards fetal biometry measurements were utilised⁶ and the Hadlock A formula¹⁴ was one of the methods being assessed. Data from other formulae evaluated in the studies was too extracted, though only the methods incorporating the AC measurement and at least one other parameter (HC or FL).

The master calculations of accurateness sought in each study were mean percentage error, relating to systematic fault and standard difference of fault, indicating random error.¹¹ The results extracted from the unlike formulae were compared to make up one's mind the most accurate and reliable method of calculating ultrasound EFW. Additional data regarding possible sources of inaccuracy was also obtained.

Search strategy

In December 2015, 4 databases were searched; MEDLINE, CINAHL, EMBASE and the Cochrane Library; searches included studies from January 2005 to December 2015, identifying all enquiry published since the previous review.^eleven In addition, the reference lists from selected relevant papers were reviewed to identify further studies.²⁵

Both subject headings and free text terms were utilised;^25,26 the search terms were as follows: [ultrasound OR sonograph* OR ultrason*] AND [f*etal weight OR EFW or f*etal biometry] AND [nativity weight OR birthweight] AND accura*. Unpublished studies and grey literature were not included,^25,26 due to fourth dimension constraints and applied issues.

A supplementary search was performed in February 2016 to identify any recently published literature; though several new articles were identified, all were subsequently excluded.

Screening and selecting

The articles were screened manually past the reviewer (first author) utilising a data screening tool; the titles and abstracts of the studies were initially assessed, to place duplicate results and exclude any research that was noticeably irrelevant.^25,26 Subsequently, total text papers of the remaining eligible studies were obtained and assessed for final inclusion.^25,26

A flow nautical chart illustrating the search strategy and reasons for exclusion was produced to preserve the transparency of the review (Figure 1).^25,26

Search strategy flow nautical chart.

Quality assessment

The quality cess of diagnostic accurateness studies (QUADAS) 2 tool²⁷ was used to assess the quality of the articles for the risk of bias and applicability,^25,26 considering assessment of sample size, data collection catamenia, number of ultrasound operators and machines, alongside referenced measurement standards and formulae.

Data extraction and synthesis

Data were extracted using a customised Microsoft Excel template, and subsequently imported into Stata statistical package.²⁸ The information were initially analysed collectively and so split into subgroups, facilitating closer comparison of specific formulae. Forest plots were produced to demonstrate the heterogeneity of the results and determine if a meta-analysis was appropriate. The chi-squared exam and I ² statistic were utilised to assess for the presence and degree of heterogeneity.^25,26

Additional descriptive data detailing sources of inaccuracy was recorded and synthesised.

Results

Table 1 illustrates the vii studies and associated methods included in the results assay; in total, 11 different formulae were assessed. Each formula incorporated the AC measurement and at least one other parameter; all studies included the Hadlock A formula.¹⁴

Tabular array 1.

Studies and formulae included in the review

Study ID	Formula ID	Parameters
1. Mirghani et al.18	1a. Hadlock A14	H, A, F
	1b. Woo et al.29	A, F
ii. Anderson et al.30	2a. Hadlock A14	H, A, F
	2b. Weiner A31	H, A, F
	2c. Weiner B31	H, A
	2d. Hadlock B14	A, F
	2e. Ferrero et al.32	A, F
3. Siemer et al.33	3a. Hadlock A14	H, A, F
	3b. Hadlock Bxiv	A, F
iv. Melamed et al.fifteen	4a. Hadlock A14	H, A, F
	4b. Combs et al.34	H, A, F
	4c. Ott et al.35	H, A, F
	4d. Hadlock et al.36	H, A
	4e. Jordaan37	H, A
	4f. Hadlock14	A, F
	4g. Woo et al.29	A, F
	4h. Warsof et al.38	A, F
v. Barel et al.16	5a. Hadlock Afourteen	H, A, F
	5b. Combs et al.34	H, A, F
	5c. Hadlock B14	A, F
	5d. Warsof et al.38	A, F
	5e. Woo et al.29	A, F
6. Faschingbauer et al.39	6a. Hadlock A14	H, A, F
	6b. Hadlock B14	A, F
seven. Rashidtwoscore	7a. Hadlock Afourteen	H, A, F

Overall, the methodological quality of the seven included studies was relatively poor. Only iv studies provided referenced standards for measurement planes,^xvi,18,33,39 and only two studies reported the number of ultrasound operators utilised, both exceeded xx, introducing inter-observer variations.^33,39 Similarly, only iii studies discussed the ultrasound equipment utilised;^xviii,30,33 just ane study used the same piece of equipment throughout,¹⁸ again suggesting a lack of consistency and inducing fault. All studies did, still, report their inclusion criteria and detailed the referenced formulae included. Equally, none of the studies made an aligning for the scan-delivery time discrepancy, reducing the possibility of bias.^eleven,20

Initially, the extracted data for all xi formulae, mean percent error (MPE) and standard deviation (SD) of error, were assessed and presented collectively (Effigy 2).

Forest plot illustrating hateful per centum mistake, standard deviation and weighting for all formulae.

The MPE varied from −six.88% (formula 2e) to 22.16% (formula 2c), and in the bulk of formulae (68%), the EFW was overestimated. In that location was a wide range in the levels of random error between formulae, with the SD varying from 7.5% to 17.78%. Formulae 2b and 2c, and 4g and 5e produced the largest levels of both systematic and random error, with MPE ranging from 17.0% to 22.16%, indicating meaning overestimation of fetal weight.

Results from virtually of the formulae (formulae 4a–4f) assessed in Melamed et al.'due south study,¹⁵ consistently produced an MPE of less than 5%, though the Hadlock A formula (4a), had the smallest SD of error (7.5%).

Due to the inclusion of differing formulae amongst the studies, a meta-analysis incorporating all results was not appropriate, confirmed in results from the chi-squared and I ⁱⁱ statistical tests; the chi-squared examination produced a p-value of <0.1 and I ² test issue was fifty.2% (Effigy two), indicating a moderate degree of statistical heterogeneity.

The results were afterward broken down into subgroups; the Hadlock A formula was investigated initially, pooling information from all 7 studies (Figure 3).

Forest plot illustrating mean percentage error, standard deviation and weighting for Hadlock A formula.

The EFW was overestimated in four studies using the Hadlock A formula. No meaning difference was identified in the levels of systematic error between older and more than contempo studies, though the random fault has reduced to below x% in the most recent four studies. When the results of all seven studies are pooled, the overall effect produces a MPE of 0.14%, indicating a slight tendency to overestimate fetal weight; the SD was 3.5%. Consistent overlap in the level of mistake was demonstrated between all studies, thus the results were combined in a meta-analysis. Chi-squared produced a p-value of 0.5 and the I ² outcome was 0% (Figure 3), indicating that there is no significant statistical heterogeneity within the results, and thus any difference within the results is probable due to gamble alone.

Data were so subsequently grouped for the formulae that incorporated two parameters, firstly, the 3 formulae including the HC and AC measurements were assessed (Effigy 4).

Wood plot illustrating the mean percentage error, standard divergence and weighting of formulae incorporating the head circumference and intestinal circumference parameters.

Results for formula 2c were the least accurate, significantly overestimating fetal weight with a MPE of 22.xvi%. Formula 4d produced the almost accurate results, with MPE of −1.5% and random error of 9.2%. Once once again, results from the I ^two statistic (66.1%) and chi-squared test (p = 0.05) (Figure iv) indicated at that place was moderate-high statistical heterogeneity within the results.

Formulae incorporating the Ac and FL were so assessed; in full six different methods were evaluated (Effigy 5).

Forest plot illustrating the mean pct mistake, standard deviation and weighting of formulae incorporating the intestinal circumference and femur length parameters.

Results were inconsistent and unreliable, with a frequent overestimation of fetal weight. The formula derived by Woo et al.²⁹ (formulae 1b, 4g and 5e) produced the well-nigh inaccurate results, with a MPE ranging from 14.7% to 20.viii%, and SD of 10.5% to xiv.5%. The most accurate results were produced by the Hadlock B formula (formulae 2d, 3b, 4f, 5c and 6b); such formula was assessed in five studies and produced mean systematic errors ranging from −4.35% to 7.4% and random errors of 8.8% to 17.78%.

Results from the I ⁱⁱ statistic (55.3%) and chi-squared test (p = 0.01) (Figure 5) indicated there was a moderate caste of statistical heterogeneity inside the results. When the results were combined, the overall MPE was vi.99%.

Sources of inaccuracy

Six studies provided information regarding possible sources of inaccuracy; three suggested the accuracy of EFW reduced in late gestation, with more difficulties obtaining accurate measurements.^16,xviii,39 Sources of error in other studies were operator focused; lack of experience, insufficient preparation and poor optimisation of the ultrasound image.^30,33 One study¹⁵ believed inaccuracy was related to the number of parameters incorporated inside the formula.

Discussion

Ultrasound adding of fetal weight is usually overestimated in comparing to actual weight.^{sixteen,18,30} Whilst the method is reliable, confidence in the accuracy of the calculation remains hindered by random mistake;^30,39 accuracy is primarily related to the formula utilised and number of incorporated biometric parameters.^15,17

The Hadlock A formula¹⁴ produced the most consistent mean systematic error and lowest random error beyond all seven studies (Figure two). When the results were pooled (Figure 3), the formula produced a MPE within the 5% level of accuracy sought in practise,^11,23 indicating an improvement in accuracy since the previous review.^xi The weighting of the studies was, yet, related to the accuracy of the results just, with no consideration of sample size or population characteristics,²⁵ limiting the significance of the results. The most accurate results for such formula were obtained past Rashid,⁴⁰ though this study had the smallest sample size (n = 73) and data were collected from the Bangladesh population only, reducing the ability and generalisability of the results.^11,18 Yet, it must be considered that babies from such population are constitutionally smaller,¹ thus highlighting the effectiveness of the Hadlock A formula¹⁴ when calculating the EFW of small fetuses, a concern raised in previous research.^12,twenty

The Uk report past Anderson et al.,³⁰ produced the highest level of error for the Hadlock A formula.^fourteen Although the study was published in 2007, the information are quondam; collected from scans performed in 2000.^thirty In the terminal decade, in that location take been significant developments in both ultrasound equipment and practice,^5,41 and thus, such results offer a poor representation of current accuracy.³⁰

The accuracy of ultrasound calculation of EFW was highest in the formulae that incorporated all 3 fetal biometric parameters; underpinning previous literature.^16,42 Bated from the Hadlock A formula,¹⁴ two other methods were assessed that incorporated all iii measurements – Ott et al.³⁵ and Combs et al.³⁴; both produced credible results (Figure 3). Comparable findings were acquired in a previous review¹¹ for the Combs et al.³⁴ formula, nevertheless, such method is volumetric rather than the typically used regression equation, and has not been widely assessed, thus the reliability of the method remains indeterminate.^{11,sixteen,42} The Ott et al.³⁵ formula was but assessed in one study within this review,^fifteen and though the results were promising, previous published literature illustrates significant inconsistency.^twenty,43

Methods incorporating the HC and Air-conditioning parameters but, performed poorly, with large random errors (Figure four).^15,30 The Hadlock et al.³⁶ formula was the most authentic,¹⁵ though random error levels remained substantially higher than those produced by the Hadlock A formula,¹⁴ echoing previous findings.^xi,twenty Calculation of the EFW based on the HC and Air conditioning parameters only, is especially rare in clinical practise, equally a reliable FL measurement tin commonly exist obtained throughout the third trimester.⁴⁴

Results for the formulae including simply the AC and FL measurements (Effigy 5) were extremely inconsistent, somewhat concerning as this method is more oftentimes used in late gestation, when an accurate measurement of the fetal head is often restricted past its deep position within the maternal pelvis.^42,45 Most significantly, across the seven included studies, random error levels for all formulae incorporating 2 parameters were persistently higher than the Hadlock A formula.¹⁴

When considering potential sources of inaccuracy, three studies suggested the accuracy of ultrasound EFW reduced in late gestation, with difficulties obtaining acceptable measurements of the fetus.^xvi,xviii,33 This issue has been widely discussed and is axiomatic amongst previous literature,⁴⁵ though more than recent studies oppose this principle.^17,46 Rosati et al.⁴³ and Dimassi et al.¹⁷ believe this source of inaccuracy is relevant when considering the high nascency weight population and fetuses with macrosomia; authentic weight prediction is critical in this population to facilitate appropriate intervention in pregnancy to reduce the take chances of complications in delivery.^17,43

Melamed et al.^xv ended the accurateness of the adding was positively related to the number of parameters within the formula, findings supported within wider literature.^17,twenty Conversely, when assessing LGA fetuses specifically, Hoopmann et al.²⁴ and Rosati et al.⁴³ believe the formulae based on the Air-conditioning measurement only, perform nearly accurately. Such authors suggest a range of formulae should exist utilised in clinical practice, and a specific method should exist called dependent upon the fetal population being assessed.^24,43

The remaining sources of inaccuracy identified were operator focused; lack of feel, bereft preparation and audit and poor optimisation of the ultrasound prototype.^30,33 Such findings are unsurprising in the current sonography climate; in the United kingdom, poor recruitment and retentiveness of sonographers has resulted in increased employment of agency staff, with little time allocated to uphold audit and training.⁴⁷ Both the U.k. Association of Sonographers⁴⁸ and the Royal College of Radiologists,⁴⁹ consider audit a primal factor in supporting and retaining skills and development, to enable competent practice and provision of a refined ultrasound service.⁴⁹

None of the studies indicated that image quality influenced the accurateness of ultrasound, a cardinal finding evident in the previous review performed by Dudley.^eleven During the last decade there take been substantial technological advancements in ultrasound equipment, and the introduction of both harmonic and compound imaging has proved highly influential on image contrast and resolution, enabling more accurate placement of calipers when performing fetal biometry measurements.⁴¹ In spite of this, the ascension levels of obesity within the maternal population must exist acknowledged,^l equally increasing BMI detrimentally affects ultrasound image quality;⁴¹ careful and objective interpretation of ultrasound findings is essential, ensuring appropriate limitations are acknowledged.¹

Limitations

Due to time restrictions, merely the first author completed the data analysis and synthesis process, thus the review is susceptible to researcher bias;^25,26 such effects were minimised by utilising a pre-specified research protocol.^25,26

In add-on, despite the Hadlock A formula¹⁴ incorporating the HC measurement, the formula was introduced over ii decades ago, and thus the standards for the measurement plane are based on old exercise and do not conform to current national screening standards;^5,9 this is a consistent, inherent upshot in all 7 included studies and wider literature, which must be addressed in future research.

Unfortunately, more recent methods for calculating EFW, including 3D formulae, could non be effectively evaluated; such techniques are still existence investigated and thus there is insufficient published research bachelor to systematically assess within the scope of this review.

Determination

During the last decade, accurateness of ultrasound calculation of EFW appears to have increased, with contempo studies consistently producing random errors below 10%. Accuracy of the adding is attributable to the incorporated parameters, with the greatest accurateness obtained when utilising all three measurements. In clinical exercise, if all three measurements cannot be obtained to the standards stipulated by the national screening committee, it is disquisitional that this is documented on the formal report, to ensure the results can exist interpreted with appropriate caution.

The Hadlock A formula remains the most reliable regression method, producing the smallest random errors. Although the volumetric formula by Combs et al. produced promising results, further evaluation is required to determine whether such method is suitable for utilise in clinical exercise.

The most common sources of inaccuracy identified were operator dependent, highlighting the importance of regular preparation and audit, key to professional development and maintaining competency.

Ultimately, ultrasound calculation of EFW will ever take inherent flaws; minimising the level of random fault volition undoubtedly improve confidence in the calculation and the clinical value of the method within obstetric direction.

Annunciation of Conflicting Interests

The author(s) alleged no potential conflicts of involvement with respect to the research, authorship, and/or publication of this commodity.

Funding

The author(s) received no financial back up for the inquiry, authorship, and/or publication of this article.

Ethical approval

Not applicable.

Contributors

JM performed the systematic review as part of work submitted for an MSc accolade at the Academy of Leeds. JA provided guidance during the review and assisted during the editing process. All authors approved the final version of the manuscript.

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Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5810856/

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