Structural problems. Problems in the structure of the ovaries (such as ovarian cysts) or other reproductive organs can cause changes in a woman's ovulation schedule.
General medical problems. Problems that affect the metabolism (such as thyroid problems) can affect a woman's ovulation schedule. So can serious illness, emotional stress, overweight and low weight, and getting too much exercise.
Cancer treatment. Exposure to certain kinds of chemotherapy or radiation can damage the egg supply in the ovaries.