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.