What the Research Really Says About ACL Injuries and the Menstrual Cycle

By Jessica Bryant

As ACL injuries continue to rise in women’s soccer, attention has increasingly turned toward the menstrual cycle as a possible risk factor. Studies spanning more than two decades—from early NIH research to more recent university-led analyses—have suggested that hormonal fluctuations may influence ligament laxity and injury risk during certain phases of the cycle. Yet despite growing interest, the science remains fragmented, shaped by small sample sizes, varying methodologies, and inconsistent hormone measurement. This article examines what current research actually tells us, where it falls short, and why caution is necessary before drawing definitive conclusions about the relationship between ACL injuries and the menstrual cycle.

Last June, FIFA began funding a year-long study at Kingston University to examine a potential link between ACL tears and the menstrual cycle. The investment reflects growing concern in women’s football, where athletes are estimated to be two to six times more likely to suffer ACL injuries than men. Some estimates suggest the risk may be even higher.

A 2002 study published in the Journal of Athletic Training found that ACL injuries were more likely to occur immediately prior to or during days one and two of the menstrual cycle, based on salivary hormone testing and self-reported cycle data. In that same study, 26 of 37 athletes sustained injuries during the follicular phase. Because the follicular phase begins on the first day of menstruation and continues until ovulation, these findings highlight how different parts of the same phase may carry different levels of risk.

Other research has pointed toward increased vulnerability during the pre-ovulatory window, when estrogen levels rise toward their peak. Elevated estrogen is believed to influence collagen metabolism and ligament laxity, which could potentially affect knee stability.

A 2023 study from Oklahoma State University reported a correlation between increased ACL laxity and the time surrounding ovulation, when estrogen levels are highest. However, increased laxity does not necessarily translate directly to injury, underscoring the complexity of isolating hormonal influence from other factors such as biomechanics, fatigue, and training load.

Taken together, the research suggests hormonal fluctuations may contribute to ACL injury risk, but it does not identify a single high-risk phase of the menstrual cycle. Variations in study design, hormone measurement methods, and definitions of injury risk continue to limit definitive conclusions.

Further research must account for additional variables often overlooked, including contraceptive use (oral, injectable, or implant-based), gender diversity, and external factors that influence hormone levels such as stress, recovery, and match congestion. Until these factors are better understood, attempts to attribute rising ACL injuries in women’s football to the menstrual cycle alone risk oversimplifying a multifaceted problem.