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Identifying small for gestational age preterm infants from the Finnish Medical Birth Register using eight growth charts

Foetal growth restriction (FGR) is a major risk factor for infant mortality and morbidity and adverse lifelong developmental, mental and physical health outcomes.1 It is usually studied using small for gestational age (SGA), which is most commonly defined as a weight under the 10th percentile based on growth charts. Identifying infants with SGA is necessary for research on the longterm consequences of perinatal pathologies and particularly important for children born preterm. Earlyonset FGR, which has been associated with preterm birth, incurs higher risks than lateronset FGR and is a prognostic factor for poor cognitive development, respiratory morbidity and metabolic disorders.2 Growth charts to identify SGA infants use different conceptual approaches and methods, reflecting ongoing debate on three key questions. First, should charts be constructed from birthweights or foetal weights estimated from ultrasound measurements (abdominal and head circumference and femur length)? Preterm infants are more likely to have growth anomalies and charts established from birthweights will have lower percentiles than intrauterine charts established from estimated foetal weights in ongoing pregnancies. Therefore, the 10th percentile will be lower and some infants at risk for complications due to poor growth may be missed.3 Second, is it possible to develop universal standards for healthy growth or should charts be based on national references? Third, do customised charts, which adjust growth trajectories for individual maternal and foetal factors that physiologically affect growth, including sex and maternal height, provide added value? Our aim was to describe the differences in SGA classifications when using national and international intrauterine and birthweight charts in Finland. We included live infants born with a birthweight ≥500 g, or a gestational age (GA) ≥22 weeks, recorded in the Finnish Medical Birth Register in 2006– 2016. The data collected included birthweight, GA in weeks and days, calculated from last menstrual period and confirmed by ultrasound, sex and variables for deriving customised charts (maternal height, prepregnancy weight, parity and smoking). We used two SGA definitions: birthweight <10th percentile and <2 standard deviations.4 We identified 37 257 preterm births before 37 weeks of GA. There were 26 891 once we had excluded births <24 weeks, multiple births, missing birthweights or GA and discordant birthweights for GA (<300 g or Zscore <6 or >4 standard deviations). Our literature review identified the eight growth charts most commonly used to examine intrauterine growth and health: two intrauterine standards (World Health Organization and Intergrowth intrauterine), two intrauterine references (Maršál and Hadlock), one birthweight standard (Intergrowth birthweight), two birthweight references (Sankilampi and Fenton) and the customised GROW chart (Table S1). Most are sexspecific, except Intergrowth intrauterine and Hadlock; Sankilampi has separate charts for primiparous and multiparous births. We used each chart to calculate the percentage of preterm SGA infants by GA, grouped in 2week categories. The proportions of preterm SGA births were Fenton (7.6%), Intergrowth birthweight (9.3%), Intergrowth intrauterine (11.8%), customised (17.2%), Hadlock (20.3%), Sankilampi (21.8%), World Health Organization (22.3%) and Maršál (22.6%). SGA births varied between charts and across gestations (Figure 1) with most variation at 28– 29 weeks: 9.1%– 29.8% for the birthweight charts, 31.3%– 47.7% for the intrauterine charts and 33.9% for the customised chart. The absolute differences between the foetal and birthweight charts were highest at these GA, then declined. The proportions after 34 weeks ranged from 13.8% to 20.9%, except for the Fenton (7.3%), Intergrowth intrauterine (8.5%) and Intergrowth birthweight (8.6%) charts. The Fenton and Intergrowth birthweight charts systematically provided lower SGA proportions across GA than the other charts. Patterns were similar using the threshold of 2SD, with a range of 1.9%– 11.2% (Figure S1). There were large differences in how the eight charts classified SGA, particularly for the intrauterine and birthweight charts at 28– 32 weeks of GA. This may reflect more indicated deliveries for earlyonset FGR at these gestations. We also observed differences between national and international charts, as Fenton and the Intergrowth charts provided lower SGA proportions than the Nordic charts. This suggests that the international charts may not accurately describe Finnish birthweight distributions. Other studies have also shown that Intergrowth charts underestimated SGA in Nordic countries.5 These results highlight the importance of choosing the right chart when quantifying FGR in preterm births and suggest that using the 10th percentile threshold with international or birthweight charts may underestimate how many children risk adverse outcomes due to growth impairment, in particular, those born at 28– 32 weeks of