Background: Recent registry studies have demonstrated a higher prevalence of asthma among women with polycystic ovary syndrome (PCOS). We aimed to assess the association and heritability of PCOS and asthma in a Danish twin cohort.

Methods: Data for 32 382 female twins from the Danish Twin Registry were included. Twins with PCOS were identified by searching the Danish National Patient Registry for International Classification of Diseases-10 code E28.2. Asthma was diagnosed by questionnaires.

Results: 103 (0.3%) women had a PCOS diagnosis. The risk of asthma was increased among women with PCOS compared with women without (18% versus 9%, respectively; OR 2.11 (95% CI 1.13-3.96); p=0.02). After adjustment for age, body mass index, alcohol consumption and smoking status, the risk of asthma was still increased, but was no longer statistically significant (OR 1.54 (95% CI 0.75-3.17); p=0.24). Variance components analysis showed that shared environmental factors explained 49% (95% CI 24-68%) and unique environmental factors explained 51% (95% CI 32-76%) of the susceptibility to PCOS. For asthma, 44% (95% CI 28-61%) of the variance was explained by genetic factors, whereas 25% (95% CI 11-38%) was ascribable to shared environmental factors and 31% (95% CI 26-36%) to unique environmental factors.

Conclusion: The risk of asthma is twice as high among female twins with PCOS. The individual susceptibility to PCOS is mainly due to environmental factors and not genetics.