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.

© 2023 The Authors. Published by Elsevier GmbH.