# healthy menstrual cycles are between 21 & 37 days > healthy cycles vary in length between 21 days (possible diagnosis of polymenorrhoea if shorter) and 37 days (possible diagnosis of oligomenorrhoea if longer; Long, 1990).[[(Schmalenberger et al., 2021)#^lfj3-]] ## Notes - Average 29 days - Determined by length of follicular phase - Optimal hormonal health - 25-35 days - <24 days - Lower progesterone - Or excess oestrogen - Too short luteal phase - Or anovulatory cycle - Or ovulation occurring too early - > 35 days - PCOS