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).