Purpose of review The aim of this review is to evaluate biological, life history, environmental, and lifestyle factors and exposures that cause variability in menstrual cycle length (MCL). Recent findings Recent literature has detailed a number of factors that influence MCL, with particular emphasis placed on novel environmental exposures, such as air pollution and endocrine disrupting chemicals. Summary MCL varies widely in response to intrinsic and extrinsic inputs and is a useful predictor of reproductive health and fecundability. Video abstract

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