Covariates
We also controlled a series of confounders, including age (years), race (White, Mixed, Asian or Asian British, Black or Black British, Chinese or Other ethnic group), education (age at completing continuous full time education), Townsend Deprivation index (a composite measure of deprivation based on unemployment, non-car ownership, non-home ownership, and household overcrowding), smoking statues (never, past and current), drinking (daily or almost daily, three or four times a week, once or twice a week, one to three times a month, special occasions only or never), moderrate activity (Metabolic Equivalent Task (MET) minutes per week for moderate activity), the body mass index (BMI), histories of diseases (diabetes, hypertension, asthma, emphysema and chronic bronchitis), female specific factors (use of oral contraceptive pills, history of stillbirth, spontaneous miscarriage or termination, menarche, age at menopause and length of menstrual cycle) in all models. Details of the assessment of covariates are described in Supplementary material.