Seeking alternatives to hormonal birth control, a growing number of people have been wearing smart rings that track their periods. There’s no shortage of opinions about this floating around: TikTok videos with millions of views discuss using such devices to help prevent pregnancy. Much of the interest focuses on the Oura Ring, probably the most mainstream example, but others are quickly entering the market. Samsung Electronics became the first major tech company to wade in when it announced its Galaxy Ring in July. Smart rings use sensors to monitor sleep, heart function and skin temperature—and if users opt in, those temperature readings can forecast their menstrual cycles.
Not everyone who monitors a cycle does so for pregnancy-related purposes—this approach can also help track perimenopause, for example. A spokesperson for Samsung says the Galaxy Ring isn’t intended for contraception or pregnancy planning. Oura’s chief product officer, Holly Shelton, says her company’s version isn’t intended for birth control either. But manufacturers’ intentions don’t always mesh with consumer desires and behavior or with third-party applications: the Oura Ring was the first wearable device to integrate with Natural Cycles, a Food and Drug Administration–approved birth control app. The temperature trend data that the ring collects are plugged into the app’s algorithm to calculate whether a user is probably fertile.
Interest in the Oura Ring, especially among younger women, increased after the Natural Cycles partnership and alongside a rise in women looking for birth control alternatives, Shelton says. Women in their 20s are now Oura’s fastest-growing market segment, spiking from 72,000 to 186,000 in the 12 months prior to April, according to Shelton.* And as of June, among female Oura Ring users, 32 percent in their 30s and 43 percent in their 20s are using the device with Natural Cycles. (Samsung’s ring incorporates some of this period-tracking tech, but the Natural Cycles app is only fully integrated with newer Apple Watches and the Oura Ring.)
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As new smart rings emerge and enthusiasm grows, researchers are evaluating these products’ effectiveness for reproductive care. “Wearables are changing the landscape for menstrual cycle tracking and what can be done with those data,” says Fiona Baker, director of the Center for Health Science and Human Sleep Research Program at SRI International, a nonprofit research institution.
Reproductive health specialists have been regarding smart rings with caution. Other contraception methods, such as intrauterine devices, are widely considered more effective than cycle tracking, particularly for people with irregular periods. Natural Cycles claims its product is 93 percent effective as birth control with typical use. When used perfectly, its efficacy purportedly rises to 98 percent. By comparison, fewer than 1 percent of intrauterine device (IUD) users will get pregnant in a year.
The “Femtech” Wave
Cycle-tracking smart rings, which also include products like the Femometer Smart Ring and Evie Ring, are part of the “femtech” wave—a rise in the popularity of services and products supporting women’s health. The market for these rings is projected to reach $50 billion by 2025. “There is definitely an increase in the number of devices offering menstrual tracking,” says Albert Titus, chair of biomedical engineering at the University at Buffalo. Because of their placement on the skin, smart rings are well suited for measuring heart rate and temperature, Titus says. Such relatively small devices lack smart watches’ energy-hungry bells and whistles, which allows for a longer battery life and extended data collection.
But they are not cheap. Brand-name smart rings sell for about $150 to more than $500, and additional costs such as membership fees can add up: accessing smart ring data requires smartphone apps that may charge subscription fees to unlock further advertised implications or insights. And then there’s the privacy issue, Titus notes. That’s a concern for all wearables but especially ones that track menstrual cycles and fertility, he explains—after the overturn of Roe v. Wade, experts are concerned these data could be used to identify someone considering an abortion.
Temperature Trackers
Body temperature fluctuates during menstruation, decreasing early in the cycle and rising during ovulation. Five days before ovulation, plus the day of and the day after it occurs, comprise the “fertile window”—the time when unprotected sex is most likely to result in pregnancy. Fertility awareness methods, which include temperature tracking, are between 77 and 98 percent effective at preventing pregnancy, according to Planned Parenthood. To track these temperature changes, smart rings must be sensitive enough to capture subtle rises of about 0.3 to 0.7 degree Celsius.
It’s an open question whether skin temperature, as measured by smart rings, accurately reflects the core body temperature changes correlated to the menstrual cycle. Thermometers are historically among the least accurate sensors on most wearable devices, says Trisha Andrew, who directs the Wearable Electronics Lab at the University of Massachusetts Amherst. “For a lot of folks, their core temperature does not linearly translate to a skin temperature,” Andrew says. Still, she adds, if someone has a very regular period, the changes related to skin temperature and the other data collected by a wearable might be enough to help track it.
Early research suggests several factors can sway a wearable device’s temperature readings. A user’s age, skin thickness and color and physical activities (as well as the ambient environment and sensor’s location on the body) all influence reliability, says Marie Gombert-Labedens, a postdoctoral researcher at SRI International. Any tracking algorithm developed by a wearable company must consider “all these potential confounders,” Baker says.
Yet a recent study involving Oura’s product suggests that rings—more so than smartwatches or bands—could capture wider temperature variations across menstrual cycles. Gombert-Labedens and Baker, who were part of the study team, speculate this might be linked to where rings are worn: extremities such as fingers undergo larger changes during vasodilation and vasoconstriction, which influence core body temperature. Additionally, the researchers found that only one reliable temperature point per day was enough to predict the cycle. Smaller studies also suggest that ring-measured skin temperature varies enough during cycle phases to predict menstruation—but not perfectly every time.
According to Oura, wearing its ring lets its in-app feature establish your unique body temperature trends, which are used to forecast period start dates and menstrual cycle phases. The company hasn’t published accuracy data on this, but Shelton says Oura considers the forecasts to be “incredibly accurate.” This is partly because it only uses nighttime temperatures to avoid fluctuations related to environments and activity, she explains. Meanwhile, in a clinical study presented to the FDA, Natural Cycles said it had established that its algorithm can identify when ovulation occurred based on Oura Ring temperature readings.
Can Smart Rings Work as Contraception?
Jessica Walter, an assistant professor of obstetrics and gynecology at Northwestern University, co-authored a 2024 review of 23 wearables—including two smart rings—designed to track fertility. Many of these devices were more like “fancy, expensive thermometers” than “groundbreaking technologies,” Walter says.
Walter has observed an increase in patients interested in alternatives to hormonal birth control, and she says certain wearables can help them better understand their cycle. But she cautions that temperature tracking isn’t reliable for everyone, especially those with irregular periods or certain medical conditions. Walter would also like to see menstrual-tracking wearables tested in larger, more diverse groups of people, rather than small groups of mostly white women in studies to date. “I’m glad that we’re thinking about how we might employ these technologies to help people feel empowered about their biology and build families on their own accord,” Walter says. “But we must understand that they’ve been validated in a very limited and specific patient population that might not be universally translatable.”
The best way to measure core body temperature is to take your rectal or oral temperature first thing in the morning, says Roisin Mortimer, an ob-gyn and researcher with the Apple Women’s Health Study at the Harvard T. H. Chan School of Public Health. She acknowledges this can be tedious, so she understands the appeal of a wearable device. Still, she thinks thermometer methods have more scientific support than what’s currently available for wearables. And the “gold standard” for confirming ovulation remains testing blood to evaluate levels of luteinizing hormone or progesterone, Mortimer adds.
Doctors and reproductive health scientists do see one major benefit to smart rings: data. This is particularly relevant to the study of menstrual cycles, which vary with age. As more people live with sensors wrapped around their fingers, this will “hopefully give us information we haven’t been able to get from studies before,” Mortimer says.
*Editor’s Note (8/13/24): After publication, Oura clarified user statistics provided by the company were accurate as of April 2024.