A large national cohort study exploring the association between small for gestational age birthweight and educational achievement in secondary school

Purpose Small for gestational age (SGA) birthweight is an important predictor of childhood intelligence, yet longitudinal evidence on its association with school achievement is scarce. We aimed to explore the association between small for gestational age birthweight and educational achievement in secondary school in a national linked data study of New Zealand adolescents. Methods This population study included all children born in New Zealand with gestational age, birthweight and sex information available between 1998–2005. This cohort was followed until Jul 31, 2024. Exclusion criteria were children born overseas, multiple births, and children absent from school for more than 6 months in the calendar year they would be expected to sit examinations. SGA was defined using the New Zealand Sex-Specific Population Birthweight Standard. Educational achievement was defined as the highest qualification attained in secondary school. Results The analysis included 372,519 children (n = 25,341 [6.8%] small-for-gestational-age [SGA] and n = 347,178 appropriate-for-gestational-age [AGA]). Compared with infants born appropriate for gestational age, in multivariable analysis, SGA children are at an elevated risk of achieving no formal school qualification (aRR 1.15, 95% CI: 1·08 − 1·24) compared to finishing secondary school with the maximum qualification available. This risk was greater for those with severe-SGA birthweight (aRR 1.20, 95% CI: 1·08 − 1·34). There was a statistically significant interaction with preterm birth. Conclusion SGA children are significantly less likely to achieve high school qualifications compared to their AGA peers. This risk is largest for those not achieving any formal qualification and increases with the severity of SGA. The interaction with preterm birth suggests SGA is only a risk factor for term-born children. Adjustment for confounders decreases but does not eliminate the effect of SGA birthweight.