Introduction: Age at menarche (AAM) is the point in development when the female starts her first menstrual period and is one of the first milestones in female reproductive life. A combination of genetic and environmental factors plays a role in the timing of female’s age at menarche.
Material and methods: This study’s purpose was to investigate the association between the biological sex of the co-twin and the age at menarche experienced by the female twin. Females from pairs of female–female twins (n = 12,176) and female–male twins (n = 2286) were collected through the Washington State Twin Registry.
Results: Environmental and genetic analyses were conducted. There was no significant difference between females with female twins and females with male twins regarding early, late or normal age at menarche (P = 0.87). However, the adjusted odds of early menarche among females with female twins were 35% lower than females with male co-twins (0.65, 95% CI 0.50–0.83). When using Falconer’s formula, an estimated heritability of 0.81 was found with shared (21.8%) and non-shared (40.8%) environmental factors contributing to the age at menarche.
Conclusion: Having a male twin during fetal life as a co-twin does not change the age of menarche in future life, although female–female twins have lower age of menarche compared to females from female–male pair. Both genetic and environmental factors determine the age of menarche.
Jahanfar S, Walters H. Association of twins’ sex discordance and age at menarche. Archives of Gynecology and Obstetrics. 2019;299:1023-1031.