Shared unmeasured characteristics among siblings confound the association of Apgar score with stress resilience in adolescence

AIM: We investigated the association between low Apgar score, other perinatal characteristics and low stress resilience in adolescence. A within‐siblings analysis was used to tackle unmeasured shared familial confounding. METHODS: We used a national cohort of 527 763 males born in Sweden between 1973 and 1992 who undertook military conscription assessments at mean age of 18 years (17–20). Conscription examinations included a measure of stress resilience. Information on Apgar score and other perinatal characteristics was obtained through linkage with the Medical Birth Register. Analyses were conducted using ordinary least squares and fixed‐effects linear regression models adjusted for potential confounding factors. RESULTS: Infants with a prolonged low Apgar score at five minutes had an increased risk of low stress resilience in adolescence compared with those with highest scores at one minute, with an adjusted coefficient and 95% confidence interval of −0.26 (−0.39, −0.13). The associations were no longer statistically significant when using within‐siblings models. However, the associations with stress resilience and birthweight remained statistically significant in all analyses. CONCLUSION: The association with low Apgar score seems to be explained by confounding due to shared childhood circumstances among siblings from the same family, while low birthweight is independently associated with low stress resilience.