Menstruation: What is the evolutionary or biological purpose of having periods

[ismnotwasm]

I'm posting this because as far as I'm concerned the entire thing is a choice argument.

Inside the uterus we have a thick layer of endometrial tissue, which contains only tiny blood vessels. The endometrium seals off our main blood supply from the newly implanted embryo. The growing placenta literally burrows through this layer, rips into arterial walls and re-wires them to channel blood straight to the hungry embryo. It delves deep into the surrounding tissues, razes them and pumps the arteries full of hormones so they expand into the space created. It paralyzes these arteries so the mother cannot even constrict them.

What this means is that the growing fetus now has direct, unrestricted access to its mother's blood supply. It can manufacture hormones and use them to manipulate her. It can, for instance, increase her blood sugar, dilate her arteries, and inflate her blood pressure to provide itself with more nutrients. And it does. Some fetal cells find their way through the placenta and into the mother's bloodstream. They will grow in her blood and organs, and even in her brain, for the rest of her life, making her a genetic chimera.

This might seem rather disrespectful. In fact, it's sibling rivalry at its evolutionary best. You see, mother and fetus have quite distinct evolutionary interests. The mother 'wants' to dedicate approximately equal resources to all her surviving children, including possible future children, and none to those who will die. The fetus 'wants' to survive, and take as much as it can get. (The quotes are to indicate that this isn't about what they consciously want, but about what evolution tends to optimize.)

There's also a third player here – the father, whose interests align still less with the mother's because her other offspring may not be his. Through a process called genomic imprinting, certain fetal genes inherited from the father can activate in the placenta. These genes ruthlessly promote the welfare of the offspring at the mother's expense.

http://www.quora.com/Menstruation/What-is-the-evolutionary-or-biological-purpose-of-having-periods/answer/Suzanne-Sadedin?srid=tsG0&share=1