Can a smartwatch tell you’re pregnant before a missed period?

You may have seen stories of people whose ring or watch ‘knew’ they were pregnant before they did. Wearables can pick up small changes in your body, but they can’t tell you you’re pregnant. Only a test can.

What a wearable measures

Many smartwatches and rings measure your skin temperature while you sleep, along with your resting heart rate and heart rate variability. Apple Watch, for example, uses overnight wrist temperature to improve period predictions and to estimate, looking back, when you probably ovulated.

What happens in a normal cycle

Just after ovulation, your resting (basal) body temperature rises slightly, by about 0.2 to 0.5°C. Wearables use skin temperature to spot this rise. It happens every cycle, whether or not you’re pregnant, so a raised temperature in the days before your period is due is expected either way.

What the research found

One of the largest studies so far used data from Oura ring users, covering 10,318 pregnancies. It was published in 2025 in the journal JMIR mHealth and uHealth. It was carried out by Oura’s own researchers and funded by Oura. It found:

Temperature changes were noticeable by week 4 of pregnancy. That’s counted from the first day of the last period, so around when a period would be due, not 4 weeks after conception. Temperature peaked at about 0.3°C above the person’s usual level at around week 9, then slowly fell until birth.
Night-time heart rate rose to an early peak at about week 5, more than 4 beats per minute above the person’s usual level. It dipped at around week 9, then rose again to a peak of about 10 beats per minute above their usual level at week 32.
Heart rate variability went down in a mirror image of heart rate.

What the study didn’t do matters just as much. It looked back at data from people who already knew they were pregnant, with pregnancies and dates reported by the users themselves. Each person’s ‘usual level’ was the average over the 4 weeks before their pregnancy was dated to start, which takes in a whole previous cycle. So an early rise overlaps with the normal rise after ovulation. The study didn’t test whether a ring can tell you you’re pregnant before a test can.

An earlier, smaller study by researchers at the University of California San Diego, working with Oura, looked at 30 pregnancies and also saw temperature rise after conception. Writing on Oura’s blog, the researchers said: “We are far from having a solution to early pregnancy testing.”

Why it can’t confirm pregnancy

The rise after ovulation happens every cycle, so a high reading before your period is due is expected anyway.
Lots of things affect the readings. Illness, alcohol, exercise and your sleep environment can all affect wrist temperature readings, and your temperature naturally varies from night to night.
The devices aren’t designed to diagnose. Apple says its wrist temperature feature isn’t a medical device and isn’t intended for medical diagnosis, and that Cycle Tracking data shouldn’t be used to diagnose a health condition or as birth control. Oura says its fertility features shouldn’t be used as contraception, and that wearable data can’t be used as a diagnostic for pregnancy health.

How to use it

If your device shows a change and your period is due or late, treat it as a nudge to take a test.

Some early tests can be used from 5 days before your period is due (the packet may say ‘6 days before your missed period’), but they won’t pick up every pregnancy that early. Testing from the day your period is due is more reliable, though a negative can still be wrong if there isn’t enough hCG yet. If your period doesn’t come, test again a few days later.

See how early can I take a pregnancy test? And if your device shows nothing unusual, that doesn’t rule pregnancy out either.

Using a wearable to track your cycle

Temperature tracking can help you learn your cycle and see when you’ve probably ovulated. It works by looking back, so it tells you when you probably ovulated, not when you’re about to. Apple says its estimates should be available after about two menstrual cycles of wearing the watch to sleep each night.

If you’re trying to get pregnant, you don’t need to time sex exactly. The NHS says that if you’re 39 or under and have sex without contraception every 2 to 3 days, it’s likely you’ll get pregnant within a year. Don’t rely on a wearable as contraception.

Questions people ask

Can my Oura ring or Apple Watch tell I’m pregnant?

Not reliably. Research shows temperature and heart rate change in early pregnancy, but the same changes happen for other reasons, and the devices aren’t designed to diagnose pregnancy. Take a test.

Does your temperature stay high if you’re pregnant?

In the Oura study, temperature carried on rising in early pregnancy, peaking at around 0.3°C above usual at about week 9. But a raised temperature before your period is due is normal in every cycle, so it can’t tell you on its own.

Can a wearable tell if something’s wrong with a pregnancy?

The Oura study saw different patterns in pregnancies that ended early, but the researchers said those findings are preliminary, and Oura says wearable data can’t be used as a diagnostic for pregnancy health. If you’re pregnant and worried about bleeding or pain, contact your maternity unit, an early pregnancy unit or NHS 111.

Can I use my watch to work out when to test?

Your cycle predictions can help you know when your period is due. Testing from that day gives a more reliable result than testing earlier.

When to get help

Mild cramps that ease with rest are usually nothing to worry about. But if you’re pregnant, or think you could be, get urgent advice from your maternity unit, an early pregnancy unit or NHS 111 if you have:

any bleeding or spotting, even if it’s light
tummy pain that’s severe, on one side, or hasn’t eased after resting for 30 to 60 minutes
pain in the tip of your shoulder

Call 999 if you have bleeding along with severe tummy pain, pain in your shoulder, or feeling sick, faint or dizzy, or if the bleeding is heavy (soaking a pad soon after putting it on). Also call 999 if you have sudden, sharp and intense tummy pain and you feel very dizzy or faint, feel sick or look very pale. These can be signs of an ectopic pregnancy or a miscarriage that need treating straight away.