Digital menstrual tracking and “cycle syncing” have risen in popularity as a way to optimize women’s health but may be putting patients at risk, according to a new analysis published in The BMJ.

Menstrual tracking technology may inaccurately estimate cycle phases, encourage health and exercise recommendations that are not well established in research, promote practices that can increase the chances of unwanted pregnancy, and expose users to privacy risks, the researchers said.

Cycle syncing involves adjusting behaviors such as exercise, nutrition, and social and work obligations according to phases of the menstrual cycle. For example, a woman might be encouraged to keep a light work schedule and eat more nutrient-dense foods during menstruation.

“One claim that particularly concerns me is the suggestion that women should substantially reduce exercise intensity during large portions of their menstrual cycle, for example, replacing more vigorous activity with yoga or walking,” said Margo Mountjoy, MD, PhD, a clinical professor in the Department of Family Medicine at McMaster University in Hamilton, Ontario, Canada and author of the study. “That can directly conflict with established physical activity recommendations.”

Though apps, influencers, and wellness programs increasingly promote the practice, making changes to exercise regimens to enhance performance or reduce injury risk has little evidence behind it, Mountjoy said. She also argued that there may be nutritional evidence for syncing on an individual basis, but it can’t be generalized to all women using a tracker.

Apps May Misidentify Cycle Phases

An estimated 50 million people worldwide use period tracking apps, which may or may not involve cycle syncing recommendations. However, menstrual tracking technology is not a reliable marker of where a woman is in her cycle and can lead users to make decisions based on questionable estimates, the researchers said.

Many apps estimate cycle phases by counting forward from the first day of bleeding. But ovulation timing varies, hormone concentrations fluctuate throughout cycles, and it’s common to have a cycle without ovulation.

Some devices, such as the Oura Ring, use metrics such as body temperature to determine where a woman is in her cycle.

Regarding the ring, “when people bring in their data, I frankly have a very hard time knowing what to do with any of the information,” said Anne-Marie Amies Oelschlager, MD, a professor in the Department of Obstetrics & Gynecology at the University of Washington in Seattle. “If I have a patient who has irregular cycles, I’m going to do my standard hormonal evaluation. That is more likely to give us a clear picture than what a ring noticed based on the pulse or temperature on a given day.”

Serious Risks for Women

Because hormonal birth control may interfere with ovulation, women who follow a cycle syncing method may be encouraged to stop using it, leading to increased risk for unwanted pregnancy, Mountjoy said.

Menstrual tracking also carries potential privacy risks, given that they are not consistently subject to strong health data protections and some companies aggregate or sell menstrual data. The researchers of study this argued that such information could potentially be used by employers, corporations, or governments, putting women at serious risk in states where abortion is criminalized.

They also raised concerns about menstrual tracking in sports, where some platforms allow cycle information to be shared with coaches. Such monitoring could influence training, nutrition, or eligibility decisions.

Combating Misinformation

Menstrual tracking itself can still be useful for monitoring symptoms, anticipating menstrual bleeding, and communicating information to clinicians, Mountjoy said. But she cautioned against relying on apps that go beyond tracking data to offering recommendations about exercise, diet, sleep, or health.

People may turn to social media, apps, and other online sources because they want to better understand their health and their bodies or when they feel they aren’t getting enough information from their healthcare providers, Amies Oelschlager said. That can leave them vulnerable to influence from less reliable sources.

“Part of our job as physicians is to really be a source of high-quality education for our patients,” Amies Oelschlager said. Clinicians should proactively teach evidence-based information to patients about the menstrual cycle and discuss the range of birth control options available should patients wish to avoid hormonal contraceptives.

Mountjoy and Amies Oelschlager reported no relevant conflicts of interest.

Brittany Vargas is an independent journalist covering medicine, mental health, and wellness.