Seasonal variations of menstrual cycle length in a large, US-based, digital cohort

•Digital cohort of 125,104 menstrual cycles from 17,427 participants within the US.•Modest seasonal variation of menstrual cycle length from June 2020 to July 2022.•Shorter cycles (<1/5 day) in May–Aug and Sept–Dec compared to Jan–April.•Younger participants <35°N and >40°N with PCOS had strongest seasonal trends.•No seasonal patterns in cycle lengths were found for participants above age 40.