Scientists think they might have finally found an explanation for why more than one-third of women experience heavy periods each month.

Rather than being related to hormones, a new small study suggests that low levels of a specific protein in the uterus might be to blame.

While most women lose up to 40 millilitres of blood each period, around 30 percent of women will lose as much as 80 millilitres - or more than a quarter of a cup - at least one cycle throughout their life.

That might not sound like a whole lot, but, trust me, it's definitely noticeable when you're running around trying to get on with work, school, and generally live life.

Sometimes this heavy bleeding is caused by a physical problem - such as fibroids or endometriosis - but around half the time, doctors have no idea what's going on, or how to stop it.

"Heavy menstrual bleeding is one of the most common reasons for referral to a gynaecologist," lead researcher Jackie Maybin, from the University of Edinburgh in the UK, told Jessica Hamzelou from New Scientist. "It can have a big impact on a person's quality of life."

Without a physical explanation, doctors can prescribe hormonal medications - such as the pill - anti-inflammatory drugs, or treatments that encourage clotting, like tranexamic acid. But for many women, these aren't effective, or come with serious side effects, so the best we can do is try to put up with it with minimal leakage. 

But the University of Edinburgh researchers say they've now found early evidence that a protein called HIF1, or hypoxia-inducible factor 1, might be linked to heavy bleeding.

So what is this protein that women everywhere are about to start hating? HIF1 is actually a pretty handy healing molecule. When oxygen levels drop in parts of the body, something known as hypoxia, HIF1 has been shown to activate more than 60 genes linked to tissue regeneration, and has already been shown to play a role in repairing the gut lining.

Hypoxia also happens in the uterus during menstruation, so Maybin and her team decided to investigate whether it might also be involved in repairing the lining of the uterus during women's periods - or not, as in the case of women with heavy bleeding.

To figure this out, they scooped out samples of cells from the uteruses of eight women over a month - half of whom experienced heavy periods (shout out to the brave volunteers for taking one for the team).

Examining these cells, they found that HIF1 did increase in the uterus during menstruation, as they'd predicted, but the women with heavy bleeding had much lower levels of the protein than those with regular periods.

To investigate whether there was a causatory link, the researchers then took a group of normal mice and mice that had been genetically modified to be unable to produce HIF1, and induced them to get their periods once a month, by giving them injections of oestrogen and progesterone that mirrored the human cycle.

They found that 16 hours after bleeding had stopped, the uterus of normal mice were already showing signs of repair. But the mice without HIF1 showed no signs of recovery, even within a 24 hour window.

That suggests that HIF1 might play an important role in repairing the lining of the uterus to stop the bleeding, and when it's not present in high enough levels, heavy blood loss continues.

That hypothesis is backed up by the fact that women with heavy menstrual bleeding often have longer periods too, lasting on average two extra days, Maybin told New Scientist.

She presented her findings this week at the 2016 European Society of Human Reproduction and Embryology conference in Helsinki, Finland, but they haven't been peer-reviewed, so we can't get too carried away just yet.

This study was also incredibly small, so the results need to be replicated in a much larger group before we can say for sure that missing HIF1 is linked to heavy bleeding - something that Maybin is now working on.

But it's still exciting news, because it's one of the first real leads scientists have had as to the cause of these awful, uncomfortable, heavy periods, and Maybin hopes in the future it could help researchers find a treatment.

With scientists also narrowing down the cause of terrible period pain, and getting close to developing a condom-less male contraceptives, it's a pretty promising time to be a woman. Thank you, science.