Q:

What is the evolutionary advantage of menstruation?

I was wondering why woman have fertile periods. Based on some simple reasoning I would expect that if woman were always fertile this would increase the chance of reproduction(one of the important things in life). But having menstrual periods would decrease this chance because only the period close to ovulation would result in fertilization.

So why did evolution result in woman having menstrual cycles?

A:

Human female ovaries only have a certain number of ova; none are manufactured in adulthood. Once they're gone (that's when menopause occurs), they're gone. 
You can also ask, why did evolution result in a limited number of oocytes then? This kind of second guessing of physiology is endless.
In your scenario, an ovum would need to be released every few days in order to be fertile continuously. That alone would decrease the number of years that a woman could be fertile. Since a fetus needs about nine months to gestate, I think once a month fertility is more than adequate. The earth's population is already too large, which also translates as successful reproductively.
If a fetus could gestate in a month, I could see an advantage to constant fertility. But the disadvantages would also be great. How could a woman take care of so many babies? How could she possibly feed, say, six to nine babies a year with only two breasts? (Mammals that frequently have larger litters have more mammary glands.) How could her own body compensate nutritionally not only for the burden of feeding the fetuses but also of the babies?
Asking why didn't something evolve from a teleological standpoint rarely results in a satisfying answer. If constant fertility were advantageous, maybe it would be present.

