[Associations between season of birth and age both at menarche and at menopause]

Objective: To examine the associations between season of birth and factors as age at menarche, age at menopause and reproductive span. Methods: A total of 285 186 female from the China Kadoorie Biobank, with complete data on critical variables and had menarche at 9-18 years old, were included. A total of 132 373 female with natural menopause were included for the analysis on age at menopause and reproductive span. Multiple linear regression models were used to assess the associations of birth season and the age at menarche, menopause, and reproductive span. Subgroup analyses were performed on birth cohorts and urban/rural residence. Results: Compared with the Spring-born (March, April, and May), participants who were born in Summer (June, July, and August), Autumn (September, October, and November), and Winter (December, January, and February) appeared late on both age at menarche and menopause. Multivariable-adjusted coefficients (95% CI) appeared as 0.14 (95% CI: 0.13-0.16), 0.26(95% CI: 0.24-0.27), 0.10 (95% CI: 0.08-0.12) for age at menarche respectively and 0.14 (95%CI: 0.08-0.20), 0.18 (95%CI: 0.12-0.24), 0.09 (95%CI: 0.03-0.16) for age at menopause respectively. No statistically significant association was found between the season of birth and reproductive span. The association was consistent between urban and rural residents and across the birth cohorts. Conclusions: female born in spring showed both earlier age on both menarche and menopause, compared to the ones born in other seasons. Our findings suggested that exposures in early life with some degree of seasonal variation might influence the development of female reproductive system.