A rise in your overnight temperature held over several nights, not one, is the honest signal, and a device can only show it after it happened.
Your resting body temperature sits a little higher after ovulation and stays there. That is why one warm night tells you nothing on its own: the signal people actually use is a small rise that holds for several nights in a row, read against your own normal rather than a chart built from other people.
Because the rise comes after the event, a ring or a watch is looking backwards. It can say that has probably already happened. It cannot say that is about to happen this week. Oura publishes its own peer-reviewed validation of its temperature-based ovulation detection (JMIR, 2025), and Apple states that the ovulation estimates its Watch produces are retrospective. We have not tested either device ourselves, so we do not put a number of our own on how well they work.
Every validated study we could find looked at cycles where ovulation happened. If you are on hormonal contraception, or you have PCOS, or you are in perimenopause, or your cycles are irregular, a temperature pattern is much weaker evidence, and sometimes it is no evidence at all. We would rather say that plainly than let an app imply a certainty it does not have.
Rhythm never tells you that you ovulated. It does not confirm ovulation from a device reading, and a temperature shift does not move your phase dates. Your phase is worked out from the dates you enter, on your phone, and those dates are never sent to our servers.
If you are making a decision about getting pregnant or about contraception, talk to a doctor, midwife or nurse. Rhythm is a training log. It is not a fertility tool, and nothing here is medical advice.