When Kiran Gandhi was in the final hours of preparation for her first London Marathon, she had a realisation many women will be familiar with: I'm about to get my period at a really inconvenient time.

"I remember evaluating my options: a pad wouldn't really be ideal — chafing is a real issue on a marathon course, and no man I know would run would put cotton between his balls and run 26 miles," Ms Gandhi told ABC podcast Ladies, We Need To Talk about her 2015 run.

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"I didn't have a moon cup on me at the time, and I guess a tampon was somewhat a viable option, but it didn't seem comfortable and I didn't want to have to run with one and change during the marathon course — there's no privacy.

"So, in a radical act to prioritise my own comfort, I decided to bleed freely and run

Her decision to go without a tampon — and let her menstrual blood flow freely — garnered international attention.

"I knew it would be a radical move. I knew it was combatting stigma and my own shame in my own right. But, I didn't know how powerful it would be," she said.

"It was so empowering. It wasn't uncomfortable at all.

"I was like, 'Damn! I'm running and bleeding.' Women do extraordinary things all over the world and I felt an enormous sense of power."

The Los Angeles-based musician was praised for combatting stigma and drawing attention to the millions of girls and women worldwide who do not have access to menstrual products.

"I ran with blood dripping down my legs for sisters who don't have access to tampons and sisters, who, despite cramping and pain, hide it away and pretend like it doesn't exist," she wrote in a blog post on her website.

But Ms Gandhi said her decision to 'free bleed' also drew criticism and she was labelled 'disgusting', 'unladylike' and 'unsanitary'.

"This reaction taught us two things: that period stigma runs deep and that we have a lot of work to do as a society to build together a world that is more loving and inclusive of women's bodies," she said.

Why the stigma?

There's a long history of menstrual taboos across nearly all cultures, and these continue to manifest in subtle and complex ways, Jane Ussher, professor of Women's Health Psychology at Western Sydney University, said.

"Periods [have long] been associated with dirt, and disgust, and shame, and some might say fear.

"Some would say that it's to do with everything in terms of the female body that's leaking and seeping, and anything that's coming out of us as women is seen as disgusting. So that would be menstruation, breast milk, sweat … what it is to be female is seen as abject."

"What I would say is that it is a sign of misogyny, it's a sign of positioning something that is essentially feminine as other, dirty and disgusting."

There's also the impact of religious views and teachings. Many of the world's major religions — such as Islam, Christianity, Judaism and Hinduism — refer to menstruating woman as unclean and suggest they be segregated during their period.

Even today, in some communities, women are banished to sheds during their period because of so-called 'impurity' during menstruation, despite the ancient practice being outlawed.

Keeping it secret

Given that menstruation is still overwhelming considered secret women's business, young girls are taught from a young age they have to manage it privately and discreetly.

"If you talk to young women across a whole range of different cultural groups about their major concern about menstruation, it's concealment," Professor Ussher said.

"It's about not knowing you're menstruating — the worst idea being blood been seen, which leads to menstrual shame."

This shame, Professor Ussher argues, not only affects how women feel about menstruation, but how they feel about their bodies.

"So that can impact on their sexuality, on their knowledge about the sexual body," she said.

Feminist writer and educator Karen Pickering said this shame had "massive knock-on effects" for how women understand their whole reproductive system.

"Our data shows us how little women actually understand that menstruation is just part of a cycle and that when you don't have your period, there are just as significant and dramatic changes happening in your hormonal profile," Ms Pickering said.

In her view, women would benefit greatly from really understanding the highs and lows of their reproductive cycles.

"That might mean that they will be able to look at their life and say, 'OK, yeah, maybe this week I'm going to have low energy, I'm going to need to take fewer trips to the gym and take on fewer responsibilities. But the weeks before and after it, I'm actually at the top of my game and that's maybe when I should do longer hours and ask for that promotion.'

"It shouldn't be a mystery of what's happening in your body," she said.

Ms Pickering has surveyed thousands of Australian girls and women about their period for a new project about menstruation and menopause.

"The responses time and time again when we asked, 'What would make getting your period better?', girls of school age said, 'Menstrual products where the wrappers don't make any sound'."

She found young girls learn quickly that the stakes are high when it comes to managing their periods.

"All of a sudden, there's this one aspect of their life that they really have to be responsible adults about," she said.

"The stakes are high socially, and in terms of their family, girls talked about how their relationship with their dads changed after that because he saw a responsibility to change the way he was with his daughter — maybe not have her sit in his lap.

"Just really sad things like this that little girls are very aware that the world around them is changing as well as what's happening inside them."

Her findings fit in with other research that has found women go to lengths to hide their period — from concealing tampons and pads at the bottom of their shopping basket, to putting a used pad in their handbag when there is no bin in a bathroom.

Shame and silence harms women

The situation is particularly challenging for many migrant and refugee women.

Speaking with women from countries including Afghanistan, Iraq, Somalia and South Sudan, Professor Ussher found that menarche — a young woman's first period — was often a frightening experience.

"A really high percentage of women told us they didn't know about menstruation before their first period, so they felt horror, they felt shock. Many women thought they were dying or that something was really wrong with them."

Despite not wanting their daughters to go through the same experience, many women felt too embarrassed to talk about their period, even with their own family.

"They say to us, 'I don't want my daughter to go through that, I don't want my daughter to have to go through what I went through', but they don't know how to talk about it — they feel shame talking about it," Professor Ussher said.

Cost and access still a major problem

Then there are the millions of girls and women around the world who are unable to access necessary menstrual products and adequate toilet facilities.

"In India alone, only 12 per cent of the entire population of bleeders have access to the products they need. I mean that's dire poverty and the fact that we don't talk about it is the reason why this happens," Ms Gandhi said.

What is a routine part of life for many women is a significant burden for others, which can involve days away from school or work, and risking regular infection.

UNICEF estimates that one in 10 school-age African girls does not attend school during menstruation.

"If you're bleeding from your body in sheer discomfort and you don't have the power to ask for the products you need … you will continually live in shadows, in quietude, in poverty and in oppression.

"It prevents women from reaching their fullest potential," Ms Gandhi said.

In Australia, girls in some remote Indigenous communities are stealing sanitary pads and skipping school for several days during their periods.

Research from the University of Queensland found menstruation was still a source of shame, and that young women felt forced to steal sanitary pads because packets could cost up to $10 each.

Australia's 49,000 homeless women face a similar challenge each month, with many resorting to using old socks, clothes, newspaper and even dead leaves to manage their period.

Menstrual education

Professor Ussher said better education and understanding of menstruation was key to improving health outcomes for women.

"Normalising menstruation as just a healthy, positive part of the female life cycle is really important," she said.

Ms Gandhi said although menstruation stigma was "only one of many systemic factors that perpetuate gender inequality", it was a large one we "frequently ignore".

"My run was about using shock factor to create dialogue around menstrual health and comfort so that women can start to own the narrative of their own bodies," she said.

"Speaking about an issue is the only way to combat its silence, and dialogue is the only way for innovative solutions to occur."