Health outcomes following caesarean birth- An observational study of twins discordant for mode of delivery.

Introduction: Observational studies indicate that children born by caesarean section (CS), have an increased risk of long-term adverse health outcomes such as allergy, obesity and autoimmune disorders. A small proportion of twin pregnancies have discordant mode of delivery, where the first twin is delivered vaginally and the second twin by CS. This provides a unique opportunity to study long-term health outcomes according to mode of birth. The aim of this study was to investigate if birth by CS is associated with cardiovascular-, metabolic-, autoimmune- or allergic disorders, by using a natural experiment with combined twin birth. Methods: Pairs of twins with discordant mode of delivery between 1973 and 2020 were identified by the Swedish Medical Birth Register. Adverse long-term outcomes were captured by diagnosis and medical prescriptions identified in national registers. Associations between mode of delivery and adverse outcomes were analyzed with logistic regression and reported as odds ratios (ORs) with 95% confidence intervals (CIs). Vaginal birth (first twin) was defined as the reference group. Results: 3196 twins were included. Most discordant twin births occurred from 1990 and onward (86.5%). There was no association between mode of delivery and cardiovascular- (OR: 0.88, 95% CI 0.63-1.23), metabolic- (OR 1.06, 95% CI 0.74-1.52) or allergic disorders (OR 0.99, 95% CI 0.86-1.14). Autoimmune disorders were more common after CS than vaginal delivery (4.3% vs 2.5%), OR 1.72 (95% CI 1.15-2.55). Conclusion: In twins with discordant birth modes, CS was associated with an increased likelihood of autoimmune disorders later in life, compared with vaginal birth.