Dysfunctional uterine bleeding, also called anovulatory bleeding, is any bleeding from the vagina that varies from a woman's normal menstrual cycle. The normal cycle is triggered by signals from hormones. Dysfunctional uterine bleeding occurs when the cycle's hormonal signals get thrown off. This can include alternating periods that are heavy and light, spotting or unpredictable shorter and longer cycles.
Regular monthly menstrual cycles flush out the endometrial lining, which is the blood-enriched layer of tissue that grows inside the uterus every month in anticipation of a possible pregnancy.
If ovulation does not occur, periods can be delayed, which allows the lining to grow thicker. For this reason, delayed periods are often heavy ones.
Lighter periods, or spotting between periods, may represent an endometrial lining that is unstable and leaking, either because hormonal levels don't adequately support it or because the lining may be too thick.
Other factors that can change bleeding patterns include:
Hormonal abnormalities (thyroid problems, elevated prolactin hormone)
Excessive exercise or weight loss
Stress or illness
The start of menstruation in adolescence — Regular ovulatory cycles may not develop for a few months or even years.
The end of menstruation — Dysfunctional uterine bleeding is common in the months to years before menopause.