Introduction
Fetal growth restriction is associated with short-term neonatal
morbidity (1-6), adverse neurodevelopmental outcomes (4, 7-9), poorer
adult health (10, 11), and a greater risk of stillbirth (12-15).
However, determining true growth restriction, when a fetus fails to meet
its growth potential, remains clinically challenging. The most common
approach to identify fetal growth restriction is to compare a fetus with
its gestational aged peers, using a predetermined threshold (commonly
<10th centile) to identify fetuses at
increased risk. These small for gestational age
(<10th centile) infants receive closer
monitoring during pregnancy and may be subject to timed birth to reduce
stillbirth risk (16-19). Therefore, accurate classification of fetuses
is important; yet substantial debate exists regarding which growth
standard should be used to define these thresholds (20, 21).
Some growth standards are derived from the average birthweight (or
estimated fetal weight), of all infants born (or measured) during a
given gestational week (22, 23). This means that the birthweight centile
of an infant is compared with all others born within the same seven-day
period. Other contemporary growth standards use statistical methods to
generate smoothed centile curves, which enables the determination of
birthweight centile for each individual gestational day (24-26).
Particularly in the latter part of pregnancy, a fetus can gain over 200
grams per week (22). This means that the fetal weight on the first day
of a gestational week (eg. 36 weeks +0 days) can be substantially lower
than on the final day (eg. 36 weeks +6 days). If using a growth standard
that reports centiles per week, rather than per day, infants may thus be
assigned a lower centile if measured at the beginning of the week, and a
higher centile if measured at the end of the week. The impact on
perinatal outcomes of classifying fetuses as SGA by individual days or
completed gestational weeks is important in determining which growth
standard is optimal to stratify perinatal risk, and to assist clinicians
in the interpretation of fetal size when centiles are only reported by
completed weeks.
Using a large population cohort, we aimed to first quantify the impact
that using a per week centile growth standard compared to a per day
centile cutoff would have on the proportion of infants classified as
small for gestational age. Second, we aimed to determine the
relationship between stillbirth and infants classified small for
gestational age using ‘week’ charts compared to ‘day’ charts.