This is just a summary. Abnormal menstruation is discussed in more detail elsewhere .

The reason human females menstruate , is due to the unique nature of the tissue lining the cavity inside the human womb. This lining is called the endomtrium. The way this lining reacts during the female cycle determines how heavy , how long and how frequent the menstruations are going to be.(Refer to MENTRUATION EXPLAINED ).

This lining react to the female hormones Estrogen and Progesterone. Estrogen causes it to thicken and Progesterone causes it to become vascular( the blood supply increases )and progesterone also stimulates the glands in the endometrium to enlarge . If the body produces to much or to little of these hormones it will cause the endometrium to start bleeding.

Diseases originating in the endomtrium can also cause bleeding. Certain growths like polyps ( benign growths) and carcinomas ( cancerous growths) can cause abnormal bleeding.

Growths located in the womb can also effect the lining and cause a change in the bleeding patterns(An example is the benign muscle growths of the womb called myomata ).

Abnormal bleeding can also originate from other parts of the female organs. Any abnormal , infected or injured tissue can cause bleeding. Any deviation of the normal ( for that person) menstrual bleeding pattern should investigated and diagnosed.