Commentary on ‘Which chart should be used to assess fetal growth? What if the best answer is “none of the above”?’
The most appropriate weight-for-age chart for assessing fetal growth remains controversial. Options include charts based on birthweights, estimated fetal weights and birthweights ‘customised’ to account for maternal characteristics. In this study, Ferdynus et al. (BJOG 2013;DOI: 10.1111/1471-0528.12282) add to the debate by evaluating a new option, a chart based on birthweights of newborns delivered to women without comorbid conditions linked to fetal growth restriction.
Birthweight standards (in which weight centiles are based only on weights of ‘healthy’ infants) have been argued to have greater clinical utility than birthweight references (in which weight centiles are based on weights of all births in a population) because they compare an infant's weight with that of normally grown infants, rather than merely establishing the infant's size relative to that of others in the population (Zhang et al. AJOG 2010;202:522–8). Weight-for-age standards (rather than references) are also well accepted for the assessment of paediatric growth, where charts such as the World Health Organization Child Growth Standards are derived from the weights of term births exclusively or predominantly breastfed until at least 4 months with no known constraints on growth and free of significant morbidity (deOnis et al. Food Nutr Bull 2004;25:S15–26). The chart of Ferdynus et al., which excludes pregnancies complicated by maternal conditions related to fetal growth restriction, should therefore theoretically be a better tool for identifying growth-restricted preterm infants. The study's finding that births classified as small-for-gestational-age by the new chart alone had relative risks of stillbirth and in-hospital death of 2.6 and 2.8, respectively, appears to support its value. However, several points deserve consideration before advocating the chart's use in clinical practice.
First, identifying which pregnancies were complicated by conditions that affect fetal growth is not straightforward. It is challenging to describe any delivery at very preterm ages as ‘healthy’, and it seems plausible that factors other than documented maternal comorbidities may also be causes of poor growth and preterm birth. Large sample sizes are needed to create weight-for-age charts, but large databases often lack the accuracy and degree of clinical detail needed to correctly identify pregnancies with compromised fetal growth. Rather than attempting to define and identify such pregnancies, it may be better to establish thresholds for ‘high risk’ based on the weight or weight centile of population birthweights at each gestational age, where risks of adverse outcomes become increased (instead of trying to identify a population or subpopulation in which risks become increased at the 10th centile (Boulet et al. AJOG 2006;195:1571–7).
Second, evaluating whether a new chart is ‘better’ by examining only relative risks of small-for-gestational-age has limitations. An important contribution of this study is that the authors additionally evaluated whether the new chart was ‘better’ using commonly accepted approaches for evaluating diagnostic and predictive tools: sensitivity, specificity and likelihood ratios. The results of these analyses helped to highlight an important point that may not otherwise be readily apparent: while the new chart may be ‘better’ than population birthweight or estimated fetal weight charts, it is still not particularly good. The likelihood ratios for all charts were < 5, suggesting that, irrespective of their various refinements, weight-for-age charts are poor diagnostic tools when evaluated by standard criteria (where likelihood ratios between 5 and 10 would provide moderate evidence to rule in intrauterine growth restriction, and likelihood ratios >10 would provide strong evidence). This confirms the importance of developing altogether new approaches for assessing fetal growth, such as ones that integrate weight-for-age with ultrasound and placental data (Zhang et al. AJOG 2010;202:522–8).
JAH is supported by New Investigator awards from the Canadian Institutes of Health Research and the Michael Smith Foundation for Health Research
Disclosure of interest
The author has no interests to disclose.