In June, the British long jumper Jazmin Sawyers shocked sports fans when she pulled out of a competition in Boston at the last minute. The 23-year-old Olympian later posted a note on Twitter explaining that her period had started just hours before she was due to leave for the track, and she was suffering with intense pains that meant she could barely walk. "This is something that isn't talked about enough in sport, and it ought to be," she added.
For all the scientific advances made in sport in recent years, ignorance around menstruation still abounds. It typically falls on individual female athletes to figure out how to deal with the effects of their periods.
Thankfully more women are speaking out about how their cycle affects their performance, including Chinese swimmer Fu Yuanhui. She told reporters at the 2016 Olympics that she was feeling "pretty weak and really tired" because she had her period, while British tennis player Heather Watson cited "girl things" as the reason for a disappointing result at the Australian Open in 2015.
For Georgie Bruinvels (28), Sawyers' revelation was bittersweet. A committed runner and PhD scientist at University College London researching how female athletes' iron stores are affected by menstruation, she explains: "At first I thought it was a good thing [that Sawyers was talking about it], but then I immediately asked a mutual friend for her details because I wanted to help her. I think it's crazy for athletes to get that far in their careers and train so hard, but this isn't something they're prepared for."
Periods can often act as a stumbling block for women in sport - whether it's young girls dropping out when they reach puberty, or elite athletes finding their training obstructed at certain times of the month.
Bernadette Dancy (37), an ultra runner - any footrace longer than the traditional marathon length -from Blanchardstown, Co Dublin, says her cycle has definitely impacted her performance, and she was anxious that her races might suffer during her period.
"That was very much a concern when I was doing the ultras. That would not be fun, having to worry about changing a tampon while you're running," says Dancy, a lecturer in health and exercise science and a mother of two.
"My period definitely makes me more sluggish. Especially after having children, your cycle is out of sync, and for me it's heavier. In the week running up to my period, I'm much more tired, so I will find it harder to get out the door, and I'll be dreading it from the day before. I wouldn't be able to work as hard, but once you get running you feel better afterwards."
Dancy has been running since her school days, and started doing marathons in 2006.
"Then I had my children and I thought I'd never do another marathon again because I couldn't find the time to train," she explains.
But after having her second child, she got back into running, particularly long runs. When a friend developed blood cancer last year, Dancy had the idea to organise an ultra run to raise awareness. Now based in Twickenham, England, she decided to run 70 miles from London to Portsmouth, in the hope of getting 70 people to donate blood. It gave her a taste for ultra running, and she completed a second race earlier this summer.
Working with a female running coach, she trains four times a week, and says her schedule is flexible to suit her menstrual cycle. Dancy also developed adrenal fatigue after having children, and has to be particularly careful managing burnout.
"If I know I'm due on a certain day and I'm supposed to do an integral training session, there's no way I'll do it. It's hard enough anyway, but I know I wouldn't get the quality of the session and wouldn't be able to push myself as hard, so I switch it around and do a slower, more temporal session and save the other one for later."
Bruinvels explains that there are many ways the menstrual cycle can influence an athlete's performance.
"Around the time of ovulation, when oestregen levels are really high, risk of injury is increased - ligament laxity is greater so you're more likely to get certain types of injury like a ligament injury, which is very common in football and rugby.
"There's a lot in the research about how subject metabolism varies, so your ability to utilise carbohydrates and fats varies through the cycle," she says.
"Strength has been shown to vary too, so at certain times of the month, you could lift more weight than others, and that could have real implications on what type of training you should do for optimum gains."
Bruinvels studied physiology, and became interested in menstruation when she realised how little research there was around how a woman's cycle affects her sport performance.
"I started my period when I was 11 and I was mortified," she recalls. "I was a swimmer and I didn't know what to do with myself, I thought I was diseased or something. I didn't go to school at all for the whole of my first period. I couldn't talk about it for years, until I was in my teens, and then I thought, 'this needs to be thought acceptable'."
Grainne Conefrey (28) from Rosspoint, Co Sligo, grew up playing soccer and hockey, and says she often found herself hit by sinking energy levels during her period.
"At some stages during matches, I've felt that you wouldn't be able to concentrate or you might just not feel up for it - you wake up feeling a bit 'ugh'. It was mainly low energy and feeling lethargic," she says.
Conefrey, who works at the Galway-based sports and data science company Orreco as a product development manager, is hopeful that more athletes will discuss it as women in sport become more visible.
"Right now it's a great time for female athletes - female sports have been on the TV a lot more, we've got the Rugby World Cup, the UEFA qualifiers. The more women's sport is put to the forefront, the more normalised menstruation will be," she says.
"I don't think it's anything to be ashamed of - it's part of your routine so it shouldn't be a taboo subject. It's something we're hoping people will speak more about and they won't have to use those euphemisms like 'girl things'."
Together Conefrey and Bruinvels created the FitrWoman app while working for Orreco, their goal being to help women maximise their sport and exercise performance during the menstrual cycle.
"The aim of the app is threefold. Firstly, to give people a physiological understanding; secondly, to provide training advice around it; and thirdly, to provide nutritional advice to support that," Bruinvels explains.
"I see it as a 'no excuses app' - I don't want someone to come to me and say 'oh no, the Olympic final is day seven of my menstrual cycle and that's when I always feel bad' - the aim is to come up with ways to stabilise your performance throughout the month."
FitrWoman is free to download from the App Store, and is designed in shades of blue (a refreshing change from the usual pink). Users are invited to enter the start date of their last period and how long it lasts. From that information, the app will give the length of the user's cycle and what point they're at. For each day, the app explains what is happening in the body and offers nutritional and training advice - for example, on day 15, it says more training gains are likely to come from endurance, and that you should eat healthy fats for slow release energy, suggesting a recipe for zucchini cheddar frittatas.
Former World Champion runner Sonia O'Sullivan has been introducing the app to young athletes, including her daughter Sophie (15), and says women are far more open about menstruation and sport now than when she was starting out.
"Girls just dealt with it themselves, very few would have discussed it, and it was rare someone would discuss it with their coaches. I was one of the lucky ones, I didn't have any issues," she recalls.
Now based in Melbourne, she is involved with training camps for teenage girls and finds the physiological information FitrWoman provides very useful.
"Athletes might think they're all alone, or think that most female athletes just don't have any problems. When you have examples of athletes speaking about their issues and how they worked around them, it becomes more normal and something that female athletes have to deal with," she says.
"The biggest thing for young athletes and teenage girls is to normalise it and to make them realise that it's not something that should stop you from doing sport, it's just something that's there and you can adapt and adjust to make it a little bit easier for yourself."