Natural Cycles, the first “digital birth control” to get sign-off from the Food and Drug Administration, is back. This time, it’s asking the agency to authorize integration with wearable devices like the Oura Ring. The feature could make it easier for users to use the app properly, but it doesn’t sidestep some of the existing issues with app-based birth control.

The Natural Cycles app uses daily temperature measurements and period cycle tracking to predict the days someone is least likely to get pregnant. It falls under the broad category of fertility awareness birth control methods, where people track when they’d be ovulating and use that information to figure out when they’d have the best chance of conceiving. It tends to be conflated with the rhythm method, which is ineffective, but when done properly, fertility awareness can be as or more effective than hormonal birth control pills.

App users have to manually take their temperature each day, right when they wake up in the morning. The new feature would let users pair the app with a wearable device that could take their temperature for them.

“We do see that there are some users that have trouble remembering to take their temperature in the morning,” says Elina Berglund, co-founder and CEO of Natural Cycles. “We listened to our users, and they said, you know it would be great if there will be something they could wear and measure the temperature during the night so they don’t have to remember in the morning.”

Anyone using the app to track their fertility can use the feature now through a beta program. People who use the app as birth control, which Berglund says is about 75 percent of users, don’t have access yet — Natural Cycles needs sign-off from the FDA first. Right now, the app is only cleared to be used as birth control with a basal body temperature thermometer, which tracks smaller temperature increments than a regular thermometer.

The FDA’s initial decision to clear Natural Cycles in 2018 was controversial: the app was blamed for unwanted pregnancies in Sweden and was heavily promoted on social media. Fertility awareness can be an effective method of birth control, but to use the technique, people usually have to learn to track and interpret their daily temperature and other fertility signals, like cervical mucus. Experts say it’s not easy to translate that careful tracking into an app, and it’s not a good option for everyone — like people who have inconsistent menstrual cycles or sleep schedules, for example.

Natural Cycles only uses temperature to measure fertility signals, and it doesn’t incorporate other metrics, like cervical mucus. Tracking only one signal tends to be less accurate than tracking both together, research shows. “Temperature is not the king of indicators for fertility awareness,” says Rebecca Simmons, a researcher and fertility awareness specialist at the University of Utah.

Using a wearable like an Oura Ring to measure temperature adds another layer of automation to fertility awareness. That has some drawbacks: it removes the app another step further from the body literacy that underlies fertility awareness methods. But it could help app users get a more accurate and consistent read on their temperatures, Simmons says. A wearable would measure temperature continuously, not just at one moment. It may be less thrown by changes in when someone goes to bed or if they’ve had alcohol the night before, she says.

Still, it only has those benefits if the ring is taking high-quality temperature measurements. Berglund says that Natural Cycles has completed an internal study on the Oura Ring’s temperature readings, but they haven’t published that data. A small study last year in 22 women found that temperatures measured by the ring and temperatures taken orally were comparable. But research is still limited. “There’s a long way to go before we can feel like, oh yeah, these are really able to tell us what we need to know in order to predict our fertility,” Simmons says.

The potential benefits also don’t elevate the app to the level of a more comprehensive fertility awareness approach, she says. “The benefits only outweigh the drawbacks if you’re already using this [app-based] approach, which already divorces you from that fertility literacy.”

Partnering with a third-party wearable device may also make it harder for users to understand where their data is going. If it’s just contained in the Natural Cycles app, there’s only one privacy policy and one set of terms and conditions. Adding another device to the equation brings in another set of rules. “It just creates a messier space for people to try to understand their own privacy rights than these apps,” Simmons says.

Natural Cycle’s filing to the FDA isn’t specific to Oura Ring — it’s open ended and would let the app integrate with any wearable device that measures temperature. “If we do a small internal study validating a wearable and see that it can give the same quality as the oral thermometer, then it can be used for Natural Cycles,” Berglund says.

Berglund says the company will evaluate how the wearable integration is being used and how well it works, both under the beta program and in a full rollout, assuming FDA clearance. Simmons says that’s one thing she appreciates about Natural Cycles. “They really do try to publish their work, they try to utilize the correct channels,” she says.

She’s excited about the possibility that people may have better ways to gather information on their fertility, but the research just isn’t quite at the caliber it would need to be for full trust in digital birth control, Simmons says. “We have to have bigger, longer, and better studies that ensure that we feel confident,” she says. “I think that the data are still too sparse to have a lot of confidence that the technology is where it needs to be in order to move forward.”