Comparison with results of previous studies and interpretation of results
The ability of the IG-21st standard to identify fetuses and neonates at risk of adverse outcomes has been recently challenged by several studies worldwide. Those studies have consistently reported that the use of IG-21st resulted in a lower prevalence of SGA compared with reference (31,32,34,36,49) or customized charts (31). Moreover, undiagnosed SGA fetuses are at increased risk of adverse perinatal outcomes and stillbirth (31,37,50). Importantly, similar to other reports from developed countries (37,42–44), we reported that the IG-21st chart identified fewer neonates as SGA and that the Latin American population turns to the right in the distribution percentiles within the IG-21st standard.
Another important finding is that the WHO identified an additional group of 4721 SGA babies who were not at significant risk of a low APGAR score. However, they have anthropometric features resembling intrauterine growth restriction. There have been several explanations for the discrepancy between the two standards. One explanation is that the calculation of EFW in the WHO study was based on the Hadlock formula (53), while IG-21st created a new formula based only on HC and AC.(54) On the other hand, IG-21st assumed a parametric distribution of the fetal growth trajectories under a linear mixed model. Researchers in the WHO project have used quantile regression to estimate percentiles directly and have fewer assumptions. It would be rational to assume, then, that compared to IG-21st, the aim of WHO was to be more of a reference, including pregnancies with complications. A previous study including 9409 women from the US reported limited accuracy of the IG-21st, NICHD, and WHO standards for identifying neonates at risk of adverse perinatal outcomes (including death) (49). IG-21st has been compared to customized charts reporting that IG-21st failed to detect SGA neonates, particularly among ethnic groups with larger maternal size.(31) Similar to our results, IG-21st classified fewer newborns as being below the 5th and 10thpercentiles by birth weight than WHO and NICHD standards (49).
Human body proportions are thought to be the product of environmental and gene interactions, and they are notable differences across different races/ethnicities and countries (55). The ponderal index is an indicator of leanness in neonates. Previous studies have shown that asymmetric fetal growth (characterized by a low ponderal index) reflects fetal malnutrition (56), is associated with cerebral palsy (57,58), and increases the risk of perinatal morbidity and mortality (59). Developing countries might welcome using the cephalization index due to its low cost. Based on this index, some recommendations postulated that the greater the brain weight: body ratio (the more severe the intrauterine malnourishment), the higher the chances are for suboptimal brain development despite compensatory mechanisms such as brain sparing (48). Since intrauterine adverse events might not be clinically relevant until late in child development, it is crucial to identify as early as possible those small neonates at risk for neurodevelopmental disabilities who need early life interventions. This is especially relevant in deprived environments, where these interventions can improve cognitive performance and reduce antisocial behavior at a young age (60,61).