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An understanding of the health consequences of abnormal menstrual function is an important consideration for all exercising women. Menstrual disturbances in exercising women are quite common and range in severity from mild to severe and are often associated with bone loss, low energy availability, stress fractures, eating disorders, and poor performance. The key factor that causes menstrual disturbances is low energy availability created by an imbalance of energy intake and energy expenditure that leads to an energy deficit and compensatory metabolic adaptations to maintain energy balance. Practical guidelines for preventing and treating amenorrhea in exercising women include evidence-based dietary practices designed to achieve optimal energy availability. Other factors such as gynecological age, genetics, and one’s susceptibility to psychological stress can modify an individual’s susceptibility to menstrual disturbances caused by low energy availability. Future research should explore the magnitude of these effects in an effort to move toward more individualized prevention and treatment approaches.

Evidence suggests menstrual cycle variation in the hormone relaxin may have an impact on ligament integrity and may be associated with risk of anterior cruciate ligament injury in physically active women. However, studies to date have only detected relaxin in a small number of participants, possibly due to inter-individual variability, frequency of sample collection, or analytical techniques. Therefore, the purpose of this study was to analyze serial serum samples in moderately active, eumenorrheic women to identify the proportion of women with detectable relaxin concentrations. Secondary analyses were conducted on two independent data sets. Data Set I (DSI; N = 66) participants provided samples for 6 days of menses and 8–10 days of the luteal phase. Data Set II (DSII; N = 15) participants provided samples every 2–3 days for a full menstrual cycle. Samples were analyzed via a relaxin-2 specific ELISA assay. Limit of detection (LOD) was calculated from the empirical assay data. LOD was calculated as 3.57 pg·ml⁻¹. Relaxin concentrations exceeded the LOD in 90.91% (DSI) and 93.33% (DSII) of participants on at least 1 day of sampling. Actual peak values ranged from 0.0 pg·ml⁻¹ to 118.0 pg·ml⁻¹. Relaxin was detectable in a higher proportion of young women representing a broad range of physical activity levels when sampled more frequently. Future studies investigating relaxin should consider sampling on more than 1 day to accurately capture values among normal menstruating women.