Heritability and risk factors of uterine fibroids — The Finnish Twin Cohort Study

Objectives: Our aim was to study heritability, risk factors and hospitalization for uterine fibroids. Methods: A random sample of 80 MZ and 80 DZ twins from the Finnish Twin Cohort were invited and 51% of the eligible women (n=82, 17 MZ and 16 DZ pairs, 40–47 years, mean age 43.0), underwent a transvaginal ultrasound. The entire cohort of 13 872 women was linked to the national hospital discharge registry 1972–1990. Results: Prevalence of fibroids was 66% and the average number of fibroids 1.7. The casewise concordance for being hospitalized for uterine fibroids was higher in MZ (0.31, 95% CI 0.24–0.37) than in DZ pairs (0.18, 95% CI 0.14–0.22). The proportion of variance in liability to fibroid hospitalization accounted for by genetic factors was 54.8% (95% CI 46.2–62.7%). Women with fibroids had higher body mass index (23.7 vs 21.7, P=0.0086), lower age at first birth (25.7 vs 29.3, P=0.012) and higher parity (3+ children 48.2 vs 29.6%, P=0.009) than women without fibroids. Risk ratio (RR) for fibroids in a MZ twin whose sister had been diagnosed with fibroids was 1.1 (95% CI 0.08;15), for a DZ twin 1.1 (95% CI 0.16;8.8) and for all twins 1.3 (95% CI 0.3; 6.1). Intraclass correlation for the number of fibroids was 0.24 for MZ and 0.11 for DZ twins, yielding an heritability estimate of 0.26. Conclusion: Reproductive and anthropometric factors may have at least as large role in pathogenesis of fibroids than genetic factors.