In the ethnically and socioeconomically homogeneous Danish population, we found no evidence that a single, relatively brief anesthetic exposure in connection with hernia repair in infancy reduced academic performance at age 15 or 16 yr after adjusting for known confounding factors. However, the higher test score nonattainment rate among the hernia group could suggest that a subgroup of these children are developmentally disadvantaged compared with the background population.

Schooling generally is positively associated with better health-related outcomes—for example, less hospitalization and later mortality—but these associations do not measure whether schooling causes better health-related outcomes. Schooling may in part be a proxy for unobserved endowments—including family background and genetics—that both are correlated with schooling and have direct causal effects on these outcomes. This study addresses the schooling-health-gradient issue with twins methodology, using rich data from the Danish Twin Registry linked to population-based registries to minimize random and systematic measurement error biases. We find strong, significantly negative associations between schooling and hospitalization and mortality, but generally no causal effects of schooling.

Oral cleft type rather than number and timing of anesthesia and operations associate to poorer academic performance. Although a potential neurotoxic effect due to anesthetic agents is not reflected in the data, it cannot be completely excluded.

Background-Smaller studies and many case series reports indicate that congenital heart defects may be more common in monochorionic twins than in dichorionic twins and singletons. Methods and Results-We investigated congenital heart defect occurrence in all twins and 5% of all singletons born in Denmark in 1977-2001 and followed through 2006 by linking the Danish Twin Registry and Statistics Denmark registers including the National Medical Birth Register and the Danish National Patient Register. Among 41 525 twin individuals, a total of 584 twins (1.4%) had a congenital heart defect registered in the Danish National Patient Register, whereas the corresponding numbers for singletons were 648 of 74 473 (0.87%; P<0.001; ie, a 63% [95% confidence interval, 45-82%] increased risk for congenital heart defect for twins). Patent ductus arteriosus and coarctation of aorta occurred >3 times as often in twins as in singletons, at 3.9 (95% confidence interval, 2.6-5.8) and 3.1 (95% confidence interval, 1.5-6.4), respectively. The increased occurrence in twins was also found in sensitivity analyses including only inpatients or only surgically treated cases regardless of whether preterm patent ductus arteriosus was included. We were not able to demonstrate a higher risk for congenital heart defect among monozygotic twins compared with dizygotic twins, and the congenital heart defect occurrence was also increased in dizygotic twins, who are all dichorionic. Conclusions-Congenital heart defect is more common in twins than in singletons, and the increased occurrence is not restricted to monochorionic twins. Intrauterine surveillance and a postnatal comprehensive cardiac assessment for both twins may be considered regardless of chorionicity and zygosity. (Circulation. 2013;128:1182-1188.)