Discussion
Main Findings
In a large, population-based cohort study, we compared the impact of
using growth standards with a common centile cutoff across the
gestational week (‘week’ charts) vs growth standards applying a unique
centile cutoff for each gestational day (‘day’ charts). Using week
charts, we found that an infant was almost half as likely to be
considered <10th centile if born at the
beginning of the week (+0 days) compared with the end (+6 days). In
relative terms, this meant 21% fewer infants were
<10th centile at +0 days, and 28% more at
+6 days, highlighting a substantial reclassification These same findings
were demonstrated for those classified <3rdcentile – the cohort most likely to be subject to obstetric
interventions based purely on size (30). Importantly, whether day or
week charts were used then impacted the relative risk of stillbirth
amongst small for gestational age infants born at +0 days and +6 days.
When week charts were used, the <10thcentile proportion was highest at +0 days, but the relative risk of
stillbirth lowest. This suggests that the SGA cohort at the beginning of
the week has been diluted by small, healthy infants, that have not yet
had the benefit of the seven-day period in which to grow. When day
charts were used, and equivalent proportions were considered SGA, no
significant differences in stillbirth risk were seen between +0 days and
+6 days. This suggests that a measurable misclassification is occurring
when week charts are used, which is corrected by using day charts.
The magnitude of these classification differences in SGA has important
clinical implications for those infants born, or measured, at the
extremes of a gestational week. If an infant is identified as
<10th, and particularly
<3rd centile, it is treated as increasing
the pre-test probability of that infant having true fetal growth
restriction (31). Fetal weight centile therefore forms an important part
of all antenatal surveillance regimes, including decisions regarding
time and mode of birth (32, 33). Thus, it is important that the
classification of smallness is applied consistently across the
gestational week and correlates with perinatal risk. If an infant is
born at 37 weeks +6 days, the likelihood of that infant being considered
<10th centile should not be half that of if
it was born a single day later, at 38 weeks +0 days, with the risk of
stillbirth- if small- altered by almost 50%.
Strengths and limitations
The large size of our statewide cohort has allowed us to robustly assess
the impact of using day specific growth standards on the detection of
FGR and its most important obstetric outcome, stillbirth. As the first
study to directly quantify the impact on stillbirth of using day charts
over week charts, our findings make a useful contribution to the field
by proposing a simple and effective way to improve classification of
fetuses at risk. Our study is limited by its retrospective design, and
by the need to exclude cases that did not have gestation in days
available. However, there is no reason to suspect this missing data
would have been unequal between groups, and it is therefore unlikely to
have impacted the results. Another limitation is that we were only able
to apply growth standards to a population of infants already born. This
means that we are only able to hypothesise about the potential benefit
of adjusting growth standards to day-specific cutoffs on antenatal
management decisions, informed by ultrasound estimated fetal weights.
Interpretation
There is considerable debate about which growth standard should be used
to define size in obstetric practice (20). Many aspects of
growth standards are highly controversial such as customisation on
maternal characteristics (34, 35), with unresolved debate regarding
which physiological characteristics have likely pathological influences
on fetal growth. Here, we have instead focussed on providing evidence of
benefit for one specific aspect of growth standards- the use of ‘day’,
rather than ‘week’ -charts, which appear to more accurately classify
fetal risk in both the clinical and research settings.
Although many contemporary growth standards do provide the option for an
individual centile for each gestational day (34, 36-38), others rely
only on a single set of centiles for each completed week. Several
international charts in use (23, 39), and the standards currently used
in Australia for postnatal birthweight classification (22, 40), provide
only a centile cutoff for each week. This means many clinicians still
receive ultrasound or birthweight reports derived from a single weekly
centile cutoff. Our findings provide strong evidence that the timing
within a gestational week of when the infant was measured needs to be
taken into consideration when interpreting the findings. Moreover, for
many years, there has been increasing concern about the high rates of
obstetric intervention and the potential for iatrogenic harm (41-43).
Here, we provide a simple method by which SGA classification can be
improved to correlate better with perinatal outcomes without inflating
the proportion of pregnancies considered at risk.