Your smartwatch can ping you about an irregular heartbeat. Your phone can assess your fall risk. And now, research suggests temperature trends picked up by wearables can tell you if you’re pregnant before you even think to take a test.
Looking at temperature data from 30 women who became pregnant while wearing an Oura ring, researchers at the University of California, San Diego, found that nightly maximum temperatures were noticeably higher two to nine days after sex that ultimately led to conception. Retrospectively, they showed how that temperature shift could have been used as a passive pregnancy notification — one that, for these users, would have popped up about nine days before they received a positive test.
If the idea pans out, it could prove a compelling use of wearable data, which has so far struggled to meaningfully change the course of clinical care.
“If women know that they’re pregnant sooner, they can make choices about their life that they might not know to make otherwise,” said co-author Benjamin Smarr, a professor in UCSD’s bioengineering and data science departments. That might mean taking steps like avoiding smoking and drinking during the sensitive early developmental stages of pregnancy. Detecting pregnancies sooner could also become critical if the Supreme Court repeals Roe v. Wade, removing federal abortion protections. “You might also say I don’t want a pregnancy, and if you know sooner, you know that there are more legal options open to you still,” said Smarr.
The fact that body temperature rises after conception is well-understood, as are the temperature changes that occur throughout the menstrual cycle, which some people monitor with a standard thermometer to help avoid or plan pregnancy. “Shockingly, if you pay attention to a woman and she gets pregnant, her physiology changes, and you can see it,” said Smarr. “It’s not rocket science.”
But it can be difficult to get a reliable trend with a single daily measurement. A thermometer reading can be thrown off by sleep quality, time of day, or even getting up to pee in the middle of the night. Researchers like Smarr, who has received compensation from Oura as a scientific adviser, contend that collecting temperature throughout the day allows for detection of fine-grained signals that can reflect a wide range of hormonal cycles and shifts — including those associated with pregnancy.
While the study showed that temperature could feasibly reduce pregnancy detection times by two-thirds in this small group, Smarr suggests that improvement is likely a floor, not a ceiling. The study was based on historical data from the first 30 Oura users who filled out a questionnaire, had enough consistent data, and happened to become pregnant — a self-selected group that was already fairly engaged with fertility data. On a population level, it’s unclear when women typically learn they’re pregnant, but what little research exists suggests that a passive alert could cut down the time before they find out by several weeks.
“There’s all this low-hanging fruit,” said Smarr. “Let’s pick it up.”
There’s more refining to be done, of course. Temperature-based alerts for pregnancy would need to be accurate across a much broader population than the primarily white, economically secure, and educated user base studied, Smarr said.
“Getting this to a place where it’s equitable and serves a broad population reliably, that’s super not easy,” said Smarr. “That requires all kinds of regulation and policy and an ecosystem of private partners, of which wearable companies will be one piece.”
In a follow-up study of a larger and more diverse pool of users, Smarr expects to find more variability in temperature patterns that could influence how an algorithm would identify a likely pregnancy. “We want to make sure we do some due diligence on that before we just say, ‘Hey, here’s one size fits all.’” The same caveats apply to fertility tracking; Another Oura-based study led by Smarr’s co-author Azure Grant from 2020 showed that the device’s temperature and heart rate variability could be used to generate a predictive signal of ovulation.
Another question: Is continuous temperature data truly that much better than what’s collected with a simple thermometer?
“It’s a hard question, because there really isn’t any data out there to make this comparison,” said Neta Gotlieb, a former colleague of Smarr’s at UC Berkeley who now works as the lead clinical research scientist at Oura.
The relative value of continuous data will depend on the cost of the devices, their accuracy, and measurement variability. Wearables typically capture wider ranges of skin temperature, not the body’s internal core temperature, with fluctuations that provide useful variance or frustrating noise depending on the application. The payoff will also depend on the way their signals are incorporated — or not — into clinical care. Though consumer wearables collect plenty of data that could theoretically be used in health care, it’s still rare for patients or their health care providers to be able to act on a red flag raised by a device. And because most fertilized eggs fail to implant, there’s also a concern about alerting users unnecessarily. “It adds this extra level of complexity, the ethical or emotional burden of knowing too early,” said Gotlieb.
But a passive pregnancy alert — one that could be just as effective for women trying to conceive as those for whom it’s the furthest thing from their minds — is a goal that Smarr still thinks is worth pursuing. Though Oura doesn’t provide any pregnancy or fertility-related alerts, users have reported realizing they might be pregnant while tracking temperature trends on their own.
“There’s no point in overstating that we’d replace the pregnancy test, that’s not the intention,” said Smarr. “One day would it ever? It probably would. Are we there yet? No.”
That future may become more realistic as more wearable devices start to integrate temperature measurements. Oura was the first wearable to incorporate the measurement, which led to the research partnership with UCSD (Grant has also received compensation from Oura as an intern). Watches from Fitbit, Garmin, and Amazon have since incorporated temperature, as have some wearables targeted at fertility tracking, like the Ava watchband.
And wearables companies seem primed to build up women as a customer base — and pool of research participants. Only 40% of Oura users are women, Gotlieb said, and “women’s health is one of the top areas of investment for Oura.” This week, the company is launching a study with Inception, a large U.S. fertility care provider, to study the physiological correlates of the menstrual cycle using the ring. “Because women have been historically understudied in the clinical trials and drug trials, we’re still at the stage where we’re learning about the basic physiology before we can study the applications,” said Gotlieb.