Early birth is a key factor in educational disadvantage of twins: A data linkage study.

Aim: To assess educational outcomes of twins and quantify the degree this is mediated by gestational age and other perinatal factors.

Methods: We conducted a population-based record-linkage cohort study of all live births ≥24 weeks gestation in New South Wales, Australia with a corresponding standardised school test result for grade 3 in 2008-2014. The primary outcome was whether a child met the National Minimum Standard (NMS) cut-off in literacy and numeracy domains. Robust multivariable Poisson models were used to obtain adjusted relative risks (aRRs), and mediation analysis conducted to assess contributing factors on the causal pathway.

Results: Of 351 791 liveborn infants, 10 365 (2.9%) were twins. After adjusting for maternal covariates and compared with singletons, twins had an increased risk of not meeting the NMS for all five literacy and numeracy domains (aRR 1.27-1.45, P < .001). Gestational age alone mediated up to 73% of aRRs and small for gestational age further attenuated these effects with only minimal risk remaining after adjusting for all mediators (aRR 0.94-1.07).

Conclusion: Almost all of the educational disadvantage experienced by twins, compared with singletons, is attributable to the risk associated with shorter gestational age, and partly by poor foetal growth. These findings support efforts to prolong gestation of twin pregnancies.