How come excessive bleeding during the period doesn't tear the hymen?

Janet

The hymen is a membrane that covers the vaginal opening before a woman's first intercourse. It is not, however, a solid mass of tissue; it has perforations in it. Often, the entire central portion of the hymen is open, with a thick rim of tissue all around. This allows menstrual blood to escape and tampons to be inserted without disrupting the hymen. Even heavy menstrual bleeding passes without tearing the hymen.

Rarely, the hymen is a solid wall of tissue, and this must be opened shortly after menstruation begins. If not, the blood will fill the vagina and uterus, leading to pain and -- possibly -- an increased risk of developing endometriosis.