Summary
An externally peer-reviewed BMJ analysis argues that menstrual-tracking apps and cycle-syncing advice have limited evidence for improving health or performance. It also highlights privacy risks and the potential for misleading recommendations.
An externally peer-reviewed analysis published in The BMJ on 30 September 2026 argues that menstrual-tracking apps and “cycle syncing” advice are being marketed with health and performance claims that are not convincingly supported by evidence. The authors, an international, interdisciplinary group, say the technologies can also expose users to privacy risks and misleading health recommendations.
Cycle syncing means adjusting exercise or nutrition according to menstrual-cycle phases. Many apps estimate those phases from the dates a user reports bleeding, then use the estimates to predict ovulation, fertility and later phases. The authors argue that this approach can give users a misleading sense of scientific precision.
Why phase estimates can be unreliable
A calendar-based app typically counts from the first day of bleeding to estimate when ovulation and other cycle phases occur. But ovulation timing can vary, hormones fluctuate between cycles, and some cycles do not include ovulation. The analysis says estimating phases without measurements such as hormone assays is widely recognised as imprecise. Basal body temperature, luteinising hormone tests and changes in cervical mucus can provide additional signals, but most tracking apps do not use the kind of measurement needed to determine phases reliably.
That distinction matters because the advice may depend on the estimate being right. The authors review sports-science research and argue that studies have not established a general effect of menstrual phase on performance, injury risk or health. Effects reported in research have been trivial and highly individualised, they say. A 2024 study of elite rugby players using rigorous methods to determine cycle phase found little evidence of a relationship between phase and a range of performance tests. Research on cycle-based nutrition is described as inconclusive, individualised or speculative.
The analysis also cautions against treating cycle tracking as a diagnostic tool. Irregular or missing periods can signal health issues, but simple tracking cannot identify their underlying causes. For example, early changes associated with relative energy deficiency in sport—including a shortened luteal phase or anovulation—are not detectable through basic app tracking. Painful periods linked to conditions such as endometriosis or adenomyosis also require clinical assessment rather than phase estimates alone.
Data, recommendations and practical consequences
The authors say the risks extend beyond uncertain predictions. Some apps and platforms aggregate and sell menstrual data, while protections for this information are inconsistent. Data could be misused by employers, companies or government bodies. In workplaces, tracking encouraged by an employer could disclose pregnancy or attempts to conceive before a worker chooses to share that information. The analysis also flags heightened risks where abortion is criminalised.
In sport, sharing cycle data with coaches could lead to false conclusions about an athlete’s health, training or eligibility. The authors say this can create particular risks for athletes who do not menstruate or use cycle-altering medication, including trans and intersex athletes.
Cycle-syncing advice may also affect everyday choices. Some recommendations urge people to reduce exercise intensity during substantial parts of the cycle, which the analysis says conflicts with physical-activity guidelines from organisations such as the World Health Organization. Some proponents advise stopping hormonal contraception to track a “natural” cycle, a recommendation that could increase the likelihood of unintended pregnancy.
The authors’ argument is not that menstrual experiences should go untracked. Rather, they contend that phase estimates and commercial wellness claims should not substitute for established health care, adequate nutrition and rest, or broader access to menstrual supplies and appropriate facilities. They call for stronger consumer and employee data protections, clearer labelling of how apps use data, and clinicians to recommend apps only when they meet stringent standards.