Purpose This study aimed to examine the incidence of injuries among different menstrual phases in eumenorrheic cycles and abnormal menstrual cycles in female athletes. Methods This prospective cohort study recruited female athletes involved in collegiate, semiprofessional, or professional team sports. Participating athletes self-reported their menstruation during the follow-up period. Menstrual cycles were classified as polymenorrhea, oligomenorrhea/amenorrhea, and eumenorrhea. Eumenorrheic cycles were further divided into estimated follicular phase (EFP), estimated ovulatory phase (EOP), and estimated luteal phase (ELP). Time-loss injuries and sports participation status were recorded by team medical staff. Incidence rates for each cycle and phase category were calculated per 1000 athlete-days with 95% confidence intervals (CI). Incidence rate ratios were used to compare risks among cycle and phase categories. Results Data from 316 female athletes were analyzed over a median follow-up period of 337 d. The incidence rates during eumenorrheic, oligomenorrheic/amenorrheic, and polymenorrheic cycles were 3.10 (95% CI, 2.61-3.67), 2.11 (95% CI, 1.44-2.98), and 4.84 (95% CI, 2.77-7.86) per 1000 athlete-days, respectively. Among eumenorrheic phases, the highest incidence rate was in the EOP at 5.14 (95% CI, 3.26-7.7), followed by the EFP at 3.19 (95% CI, 2.47-4.04) and the ELP at 2.52 (95% CI, 1.85-3.35). The incidence rate ratios for EOP compared with EFP and ELP were 1.61 (95% CI, 1.004-2.586) and 2.04 (95% CI, 1.238-3.357), respectively. Conclusions Injury risk is significantly higher during the EOP compared with the EFP and ELP in eumenorrheic cycles. Injury incidence was found to be lower during oligomenorrheic/amenorrheic cycles than during eumenorrheic cycles. Future studies should explore the factors contributing to the heightened injury risk during the EOP and the observed reduction in injury rates during oligomenorrheic/amenorrheic cycles.
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