Hayes and her staff have designed players’ individual plans around the phases of their menstrual cycle with the belief that factoring in the menstrual cycle to training and nutrition regimes could help control the weight fluctuations which often affect athletes during certain phases of their cycles.

This in turn can reduce a player's susceptibility to soft tissue injuries, such as anterior cruciate ligament damage, which has been linked to menstruation.

‘The starting point is that we are women and, ultimately, we go through something very different to men on a monthly basis,’ said Hayes.

‘We have to have a better understanding of that because our education failed us at school; we didn’t get taught about our reproduction systems. It comes from a place of wanting to know more about ourselves and understanding how we can improve our performance.

Last February, Hayes met physiologist and international cross-country runner Dr Georgie Bruinvels, who has developed the FitrWoman app for sports science company Orreco. The app allows women to input information about their menstrual health and related symptoms, which can then be logged and monitored.

Hayes, along with her backroom staff and other members of the management team, have encouraged players to download the app and, with the consent of the players, Chelsea’s coaches access this information and tailor their training programmes.

Dr Bruinvels visits Chelsea once a fortnight and instructs staff and players on the use of the app and how it can be applied to training to optimise performance. Players learn how to track their menstrual cycle across the four phases: menstruation, pre-ovulation, the time between ovulation and premenstrual symptoms and the premenstrual phase itself.

A player can be affected in different ways depending on the phase of their cycle. They can lose co-ordination during phases one and four and often crave junk food during phases three and four, which can lead to weight fluctuations.

Understanding more about the subject could also have a significant impact on preventing injuries, as there can be a higher injury risk during phases one and two. This ranges from serious injuries – research has suggested a link between anterior cruciate ligament injuries and hormonal fluctuations – to less severe soft tissue problems, which are more likely to occur during the first half of a cycle.‘The menstrual cycle is an inflammatory process and excess inflammation can result in an injury,’ said Dr Bruinvels ‘It’s not solely down to high levels of oestrogen, but tracking the cycle is also very important in terms of bone-injury risk.’

Talking about how this could change the game for the better, Hayes added: ‘We view ourselves, in a lot of ways, as leaders of the game and it would be amazing if others started doing this.

‘These players are going to be the first generation of women who are well educated about their menstrual cycle and they will spread that knowledge as far as they possibly can and we hope that becomes a culture within every football club in the world, so everybody can cope better with their menstrual cycles.’