Prudence Wade investigates the effect menstruation has on female athletes, and why the sporting world is so quick to ignore it.
Chinese swimmer Fu Yuanhui has already stolen our hearts at these Olympics with her hilarious facial expressions. Now, she's going even further by addressing one of sport's most taboo subjects: menstruation.
On Monday Yuanhui came fourth in the 4x100m medley relay. Asked by a reporter from CCTV (China Central Television) if her stomach hurt, she replied: "My period came last night and I'm really tired right now.
"But this isn't an excuse, I still did not swim as well as I should have."
Many were shocked, as the issue of periods is rarely discussed or considered in sport. We spoke to Toby Richards, professor of surgery at University College London, about athletes and menstruation.
"There's a huge gap," Prof Richards said. "Every other person in the country is a woman, and if you actually look at the population of women at any point in time one in 10 have iron deficiency anaemia.
"This is a consequence of normal menstrual bleeding, and yet it's regarded as so common and so normal that we ignore it."
So what impact does iron deficiency anaemia actually have? One of the major side effects is fatigue, which is obviously far from ideal when you're competing in a sporting event. Other side effects include (but are by no means limited to) restless leg syndrome, constipation and fainting - all things that can affect your performance.
If one in 10 women has iron deficiency anaemia, logic follows that a tenth of female athletes do too.
The world of sport very rarely addresses the fact that female athletes have periods, and yes, this can affect their performance. For all the ladies reading this: Would you want to compete in an athletics event when it's your time of the month, complete with bloating and stomach cramps?
Raffaella Pontonutti, sports science and therapy lecturer for the faculty of life sciences and computing at London Metropolitan University, explains how periods can affect athletes. Fluctuating levels of hormones cause "weight gain, rise in temperature, irritability and depression. There is also a loss of mental drive and physical stability resulting in slower reaction time and efficiency which could be detrimental for a woman athlete," she says.
Not only this, but "during the first two days of menstruation, there is also a decrease in oxygen delivered to the muscles that could result in early fatigue", she says. No wonder Yuanhui wasn't feeling her best for the race.
In their work, Richards and his team aim to show that athletes are women too, and their cycles shouldn't be ignored.
He said: "We've shown that the menstrual cycle is the number one thing that can impact on training and elite athletes. There's a huge misconception out there that top athletes don't have a menstrual cycle: amenorrhoea."
Richards set up the Iron Clinic, which studies top female athletes with iron deficiencies. They have the ability to give an iron injection, which can restore iron levels within days - vastly quicker than iron tablets. When female athletes with iron deficiencies were administered these injections, there was a 10% improvement in physical performance in just two weeks. He made it clear that only women with existing iron deficiencies were treated, as he does not want it to be used for doping.
The last time this topic hit the headlines was in early 2015 thanks to British tennis player Heather Watson. When Watson went out of the Australian Open in the first round, she blamed her cycle being out of sync with the tour: "I think it's just one of these things that I have, girl things."
Yuanhui speaking out is even more remarkable considering how taboo menstruation is in her native China. According to research, only 2% of Chinese women use tampons. A staggering 31% of women hadn't even heard of tampons, and 38% had no idea how to use them.
Yuanhui's fanbase continues to grow, with many people commending her bravery on the Chinese social media site Weibo.
Richards accepts that it's hard to fully know the impact that menstruation and the specific stages in a women's cycle have on her sporting ability.
"There's huge gaps in the research," he said. "The key take-home message is: Athletes are women too. Just because you're an athlete doesn't mean you don't have a period - periods, pre-menstrual tension and heavy menstrual bleeding is exactly the same as in the general population."
So where do we go from here? Those who have iron deficiency anaemia should be treated to perform to the best of their ability, and this iron injection is a brilliant way of helping sufferers.
Perhaps all we're asking for is a recognition that athletes are women too. Sometimes an athlete will be on her period, and this can affect her performance. This should be something we can address without shocking the sporting world.