Once menstruation starts, most women have regular cycles. They know when to expect bleeding and when it should end, but some women struggle with irregularity or unusual bleeding during or in between their periods. This is called "abnormal uterine bleeding."  It's described as bleeding that's heavy, lasts a long time, or happens between periods. Abnormal uterine bleeding can be caused by common hormone problems. One hormone problem is called "anovulation." This is when an ovary does not produce an egg during a menstrual cycle. This happens in many women 1 or 2 times each year. It can be more common in perimenopause. This is a long period of hormone changes leading up to menopause.