Sexual Relationships 
Sexual Health 
I've seen a lot of questions from guys on the subject of periods, and to prevent writing the same thing every time I see the question I'm just going to write this once and link to it each time. It should be clarified that by the title I'm talking about "everything I know about periods."
What is it?
A woman's period is the process in which every month (roughly) she sheds a spongy, viscous lining on her uterine wall and an egg if it has not been fertilized.
What are cramps?
Cramps help in the breaking down of the uterine lining. If a woman has a particularly thick lining, her uterus is tilted, or her cervix is small it can be particularly painful. The "pill" helps ease this pain by thinning the lining.
What about PMS?
PMS occurs because of a whole lot of hormonal changes during this time, interestingly though, it's actually mostly the drop in all hormones. Less progesterone etc
How long?
It usually occurs every 25-35 days and typically lasts 3-7 days, but can last longer for some women.
How much blood is lost?
Typically between 4-12 teaspoons of blood AND other materials.
Why does this happen?
This is actually an interesting and slightly more complex question. Periods, or external menstruation is very rare within the animal community with mostly only other primates demonstrating this phenomena. Researchers have come to an interesting conclusion that this occurs in species in which the mother is responsible for controlling whether or not she gets pregnant. Basically the fertilized egg can only attach IF she has this spongy zygote friendly lining. This ability is called "spontaneous decidualisation." Two hypotheses are that 1.) self defense in these species the fetus is quite aggressive and will burrow all the way into the blood stream if allowed and thus this process came about to defend the mother. 2.) In order to expel bad embryos.
I think it is important to add (although not really noted in the article I read) my observation that the species that demonstrate this phenomena all are ones in which have a heavy cost involved in child-rearing. Pregnancy and birth for these species are incredibly taxing and can frequently result in physical harm to the mother and/or the fetus. Additionally, after birth the mother has to expend a lot of resources on nurturing these young. Thus it is imperative that they ensure only the "best" embryos get implanted.
Cool/ Random stuff?
These are things that I'm also aware of, but don't as frequently notice as questions.
1.) Many women describe increased sexual desire during this time.
2.) Many women's breast appear to engorge during this time.
3.) A study has shown that many men find a woman's "moan" more attractive during this time.
PS I'm aware of the hilarity of a man writing about this, but whatever.
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