Background: The menstrual cycle has been hypothesized to influence injury risk in female athletes due to hormonal fluctuations affecting musculoskeletal, metabolic, and neuromuscular systems. However, methodological inconsistencies and variability in phase classification have resulted in conflicting evidence. The lack of biological samples forces researchers to estimate and speculate about the relationship between the menstrual cycle and injury risk. It is well established that each phase of the cycle is characterized by specific hormonal profiles with distinct physiological functions. Without direct measurement of these hormone levels, it is difficult to generate accurate and reliable data. The only phase in which hormonal status can be confidently inferred is menstruation, as this phase is defined by low concentrations of ovarian hormones. Understanding this relationship in elite female football players is crucial for optimizing training load, health management, recovery strategies, and injury prevention. Objective: To investigate the association between the early follicular phase (menstruation) and the incidence of musculoskeletal time-loss injuries in elite female football players across four competitive seasons. Study design: Descriptive epidemiological study. Methods: Menstrual cycle and injury data were prospectively collected from 33 elite female football players between the 2019/20 and 2022/23 seasons. Menstrual cycle was tracked using a calendar-based digital tool, and injuries were classified according to the OSICS-10 coding system. Injury incidence rates per 1,000 h of exposure were computed and compared between bleeding and non-bleeding phases. It should be noted that in this article, the terms bleeding, menstruation, phase 1, and early follicular phase are used interchangeably to refer to the same stage of the menstrual cycle. Results: A total of 852 menstrual cycles were analysed, during which 80 injuries were recorded. Of these, 18 (22.5%) occurred during matches and 62 (77.5%) during training. The most common injury types were muscle injuries (57.5%), ligament injuries (30%), and tendon injuries (12.5%). Injuries during the bleeding phase accounted for 13.7% (n = 11) of all cases. The overall injury incidence rate was 6.42 per 1,000 h (95% CI: 5.09-7.99), with an incidence of 5.46 per 1,000 h during the bleeding phase and 6.60 per 1,000 h during non-bleeding phases (p = 0.55). Although injury incidence was not significantly different between phases, the injury burden was substantially higher during bleeding (684 vs. 206 days lost per 1,000 h; p = 0.0027), indicating that injuries sustained during menstruation resulted in more severe consequences. Conclusions: This study provides evidence that injury incidence is similar during menstrual bleeding compared to non-bleeding days in elite female football players. However, injuries occurring during menstruation are associated with a significantly higher burden, suggesting an increased risk of more severe injuries during this phase, these findings highlight the importance of individualized menstrual tracking for injury prevention and athlete health management. Further research with precise hormonal monitoring is needed to confirm these observations and to inform training, recovery, and health strategies in female athletes.
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