If you're over 45 and you're pushing past 35 days, it may be because you're perimenopausal and beginning to ovulate less frequently. According to another recent study, as women age from their 20s through their early 40s, the average menstrual cycle declines from 30 to 27 days (though many women had cycles that were longer or shorter). This average increases as you approach menopause: up to 35 days when you're three to four years away, then 45 days two years prior to the change, and finally 80 days in the year before you stop altogether.
Talk to your doctor if you're younger than 45. Also if you're suddenly vacillating a lot more than is typical for you. "If you have a 14-day cycle followed by a 36-day cycle, that's usually an indication that you have a hormone imbalance," says Mary Jane Minkin, MD, clinical professor of obstetrics and gynecology at Yale University School of Medicine. You should have your ob-gyn check for an underlying cause, especially if you're still at an age where you might want to get pregnant.