As a woman, all I want to do is embrace body positivity, but periods make this a very difficult thing to do. Every month our endometriums (the lining of the uterus) decide that the best thing to do is flood out of us, forcing us to buy bits of expensive material to put into ourselves. Plus we get to experience a range of minor (and sometimes major) physical and emotional side effects. Seriously science, why do we have to go through this so many times a year?!

The short answer is that the science is still annoyingly unclear on this point. There are many theories, which have all been rebutted by many others. Most fall under the idea that periods were once useful, but are now out-dated. Another, considered the most reasonable, is that it is more cost-efficient to shed the endometrium than to continually maintain its health until pregnancy or menopause.

This is a very narrow view. Out of all mammals, only a few (us, some other primates, bats and the elephant shrew) have periods. Luckier animals only build upon a base layer of endometrium in response to fertilization, or they get to reabsorb their endometrium. This seems like a much better option – no loss of iron or other nutrients that we’ve managed to save for a month, no accompanying mood swings or other side effects.

And why do these side effects occur? PMS is probably the most well known symptom of changing hormones associated with menstruation, and it can make many people feel sad, tired, anxious and irritable. I find this the most frustrating aspect of my period. It has me questioning every time I’m upset whether my feelings are legitimate or overhyped, something I shouldn’t have to think about. What’s less well known is that PMS can last even after your period begins, and also causes tender breasts, weird bowel movements and cravings.

I’ve been lucky enough to experience cramping only a few times, but I know people that are in so much pain every month that they faint, or literally can’t get out of bed. It’s thought that cramps occur due to the uterus contracting to push out its lining, and if it contracts too hard it can cut off oxygen supply to nearby muscles. In the process it can push out a few more unwanted things, like diarrhoea and vomit. How fun!

What actually is a fun side effect is feeling really, really horny during your period. There are quite a few theories for this one, all hilarious. One is that due to all that extra fluid putting pressure down there it accidentally leads to prolonged stimulation. The other is that we want what we can’t have, although you can still orgasm via pretty much any of the usual methods (I’ll just throw some keywords around here; like vibrators, a towel, the shower…) so I’m not sure about this one. Also, orgasms can actually help cramping and PMS symptoms. And of course, the horniness could also be due to the change in hormone levels, but scientists haven’t pinpointed which hormones.

What this lack of conclusion really uncovers is that there is a big hole in the scientific understanding of female anatomy. There is still no fully effective treatment for severe cramps, no female equivalent for Viagra, and the contraceptive pill still sucks. For now I’ll just have to celebrate periods for the psychological relief they bring as a pregnancy test, and hope evolution eventually catches up to spare future generations of women.