Validity of Self-Reported Menstrual Cycle Length.

BACKGROUND: Menstrual cycle length is an important marker of reproductive and overall health; however, the validity of self-reported measures remains debated. PURPOSE: This study estimated agreement between self-reported and calculated menstrual cycle length among premenopausal participants in the Cancer Prevention Study-3. METHODS: Usual cycle length was reported using six predefined categories and calculated using prospectively reported menstrual start dates. Agreement was assessed using descriptive comparisons, Spearman's correlation, and weighted kappa statistics. Analyses incorporated a ±7-day buffer to account for expected biological variability. RESULTS: Of 55,458 participants, age range: 19-54 years old, 44.9% had exact agreement between self-reported and calculated cycle length. When applying the ±7-day buffer, 82.2% were in agreement. Correlation was weak using exact categories (ρ=0.17) but improved with the ±7-day buffer (ρ=0.53). Agreement was poor using exact categories (κ=0.15) and increased to fair when accounting for biological variation (κ=0.33). Agreement varied across subgroups, with generally higher concordance among participants aged 30-40 years, White or Asian, graduate degree, higher income, or nulliparous and lower concordance among separated/divorced/widowed participants. CONCLUSION: Self-reported menstrual cycle length demonstrates fair agreement when biological variability is considered, supporting its use in epidemiologic research and providing empirical justification for incorporating in menstrual-related analyses.