A government study of more than 71,000 U.S. nurses showed those who had worked rotating shifts for at least 20 consecutive months were 23 percent more likely to have irregular menstrual cycles than those working regular or non-rotating shifts.
For the purpose of the study, an "irregular" cycle meant varying by more than a week.
Menstrual irregularities can make it more difficult for a woman to get pregnant.
The study is published in the journal Epidemiology.
