Objectives We investigated associations between nursing occupational exposures and menstrual cycle regularity and cycle length.

Method Cross-sectional data were collected in 2010–2012 from 6309 nurses aged 21 to 45 from the Nurses’ Health Study 3. We used multivariable regression modelling to analyse the associations between occupational exposures and prevalence of irregular cycles and long and short cycle lengths.

Results Cycle length was recorded as <21 days (1.5%), 21–25 days (15.6%), 26–31 days (69.7%), and 32–50 days (13.2%). In addition, 19% of participants reported irregular cycles. Working more than 41 h/week was associated with a 16% [95% confidence interval (CI): 4–29%] higher prevalence of irregular cycles and a higher prevalence of very short (<21 day) cycles [prevalence odds ratio (OR) 1.93, 95% CI: 1.24–3.01]. Irregular menstrual cycles were more prevalent among women working nights only (32% higher) or rotating nights (27% higher), and their prevalence was associated with the number of night shifts per month (p for trend <0.0001). Rotating night shift was also associated with long (32–50 day) cycles (OR 1.28, 95% CI 1.03–1.61). In addition, heavy lifting was associated with a higher prevalence of irregular cycles (34% higher), and the prevalence of cycles <21 days and 21–25 day cycles increased with increasing amount of heavy lifting at work (p for trend <0.02 for each endpoint).

Conclusions Night work, long working hours, and occupational physical labour might play a role in menstrual function disturbances.