Twins as a natural experiment to study the causes of mild language delay: I: Design; twin–singleton differences in language, and obstetric risks

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Abstract
Background: Twins tend to lag behind singletons in their language development, but the causes were unknown. The possibilities suggested include obstetric complications, twin‐specific features, and postnatal differences in family interaction. The present study was designed to pit these alternatives against one another as possible causal influences. Method: The Avon Longitudinal Study of Parents and Children (ALSPAC) was used to identify the 116 twin pairs (of whom 96 participated) and 114 pairs of singletons (of whom 98 participated) whose ages were no more than 30 months apart. The McArthur Communicative Development Inventory was completed at 20 months, and the Pre‐School Language Scales (PLS‐3), and the McCarthy Scales of Children's Abilities at 36 months. Obstetric and perinatal complications were assessed on the basis of detailed systematic parental reports, together with a systematic coded abstraction of all medical records dealing with pregnancy and the neonatal period. Family background details were assessed from parental reports, and the primary carer's verbal functioning was assessed by the Mill Hill Vocabulary Scale. Congenital anomalies were assessed using the method of Waldrop and Halverson. Results: The language of twins was 1.7 months below that of singletons at 20 months and 3.1 months at 3 years. The verbal cognitive score of twins was about half a standard deviation lower than that of singletons. The twin–singleton differences in language level were found to be unassociated with obstetric/perinatal features as assessed from both parental reports and medical records, to birthweight or gestation, to birthweight discrepancy within the twin pair, or to congenital anomalies. Conclusions: It is concluded that obstetric/perinatal features do not account for the slower language development in twins as compared with singletons, within a sample born after at least 33 weeks gestation.