when i trained. but it's notjust athletes that this affects. women make choices about training around their periods all the time. sometimes, you want to rest and other times, you want to run. that's because we have different hormone levels at different times of the month. our menstruation cycle is split into four different phases. they are period, follicular, luteal and premenstrual. each phase is determined by the two main hormones, oestrogen and progesterone, which are at differing levels, depending on where in the cycle you are. using this information as a guide, jess decided the best way to get her message across to women who want to keep fit was an app. after a couple of months of inputting your period data, the algorithm begins to recognise what phase of the menstrual cycle