In 1993 Margie Profet, an evolutionary biologist from the University of California at Berkeley, announced that she had figured out a better explanation for why we menstruate than the traditional idea that the uterus lining didn't get used, so it had to go somewhere. She says that the idea that we bleed every month just because we didn't get pregnant has bothered her since she saw her first sex-ed films in school. As an evolutionary biologist, she believes that our every function should contribute in some way toward our survival as a species, and bleeding for no good reason just didn't make sense.
Her theory is that menstrution fights disease, that it keeps infections out of the uterus. In a nutshell, she believes that sperm which enter the uterus after sex carry bacteria which is potentially harmful, and could be absorbed through the walls of the uterus. During the process of menstruation, the bacteria are flushed out of the body along with the lining of the uterus, and the blood flow which accompanies the flushing also contains immune cells called microphages which attack any lingering bacteria.
Profet's theory has caused a lot of controversy in the scientific community, and it will be a while before we find out if she is correct. Even if she is proved wrong, the new attention she has brought to the often ignored subject of menstruation is valuable in itself.