Age at natural menopause is defined as the age when menstrual periods ceased permanently and naturally (in years). Questions regarding menopause status were asked in baseline and subsequent questionnaires. The questions were: “Have your menstrual periods ceased permanently?” If yes, “At what age did your natural periods cease?” and “For what reason did your periods cease?” Response categories were: “Surgical; Radiation or Chemotherapy; Natural”. Age at natural menopause was assessed in the baseline questionnaire for postmenopausal women at baseline, and updated in subsequent questionnaires for premenopausal women at baseline. By the time we conducted this study, all women in the NHS and the WGHS had passed through menopause and age at menopause was recorded, either at baseline, or on subsequent questionnaires. We used this most updated information for age at natural menopause in the analysis. Women who had radiation/chemotherapy or surgically- induced menopause were excluded from the current study. We also excluded women who reported ages at menopause younger than 40 years or older than 60 years from the analysis as these extremes might reflect underlying pathologies.
