Does basal body temperature predict ovulation or confirm it?

It confirms it. The temperature rise happens after ovulation, once progesterone climbs — so BBT tells you ovulation already occurred rather than predicting it in advance. That after-the-fact confirmation is still valuable, because once ovulation is pinned down, the timing of your next period becomes much more predictable.

Can an Apple Watch track ovulation with wrist temperature?

Apple Watch measures wrist temperature overnight, and Apple Health uses it to give a retrospective ovulation estimate. It's a convenient proxy for classic basal body temperature — you don't have to take a reading manually each morning — though it's slightly less direct. It still works on the same principle: detecting the small, sustained rise that follows ovulation.

How big is the temperature shift at ovulation?

Small — typically around 0.2–0.5°C (about 0.4–0.9°F). It's not something you'd notice by feel; it only shows up as a sustained step-up across several days when you chart it. That's why a single reading means little and the pattern means everything.

Is basal body temperature reliable for avoiding pregnancy?

Not on its own, and not from temperature alone. Because BBT only confirms ovulation after it happens, the fertile days before ovulation have already passed by the time the temperature rises. Fertility-awareness methods that work combine temperature with cervical-mucus tracking and proper training — an app's temperature reading is not contraception.

Basal body temperature and ovulation, explained

A small, sustained rise in body temperature is the clearest at-home sign that ovulation already happened. Here’s what it is, how wrist temperature fits in, and what it can — and can’t — tell you.

Short answer: after you ovulate, the hormone progesterone nudges your resting body temperature up by a few tenths of a degree, and it stays up until your next period. That step-up is basal body temperature (BBT). It can’t warn you that ovulation is coming — but it confirms, clearly, that it happened. And confirming ovulation is the single most useful thing you can do to sharpen a period prediction.

What “basal” temperature means

Basal body temperature is your body’s temperature fully at rest — classically taken the moment you wake, before getting up, eating, or even talking. At that baseline, the cycle’s hormonal shifts become visible. Through the first half of your cycle the reading sits a little lower; after ovulation it rises and holds. Chart it over a month and you see two plateaus with a step between them — the “biphasic” pattern.

Why the rise follows ovulation

Once an egg is released, the structure left behind (the corpus luteum) produces progesterone, and progesterone is mildly thermogenic — it raises your baseline. So the temperature shift is a consequence of ovulation, arriving a day or so after the event. This is the key thing people get backwards: BBT is a confirmation, not a forecast. For what the event itself is, see what is ovulation?

How it sharpens a prediction

The first half of your cycle — before ovulation — is the variable part, which is why calendars miss. But the luteal phase (ovulation to your next period) is far more consistent, usually around two weeks. So the moment temperature confirms ovulation, the countdown to your next period stops being a guess and becomes close to arithmetic. That’s why Dew, when you let it read temperature from Apple Health, can tighten its forecast: it measures the back half of your cycle instead of estimating it. The full picture is in how period predictions work.

Wrist temperature and the Apple Watch

Taking a manual BBT reading every morning is accurate but demanding. Apple Watch offers a gentler route: it samples wrist temperature while you sleep, and Apple Health turns that into a retrospective ovulation estimate. It’s a slightly less direct measurement than an oral basal thermometer, but it captures the same sustained rise without asking anything of you. Dew reads that signal from Apple Health — it’s never asked for separately, and it never leaves your device.

What temperature can’t tell you

It won’t catch the fertile days in advance. By the time the temperature rises, the most fertile days (the lead-up to ovulation) have passed. For that you need live signs — see the fertile window.
One reading is noise. Illness, a poor night’s sleep, alcohol, or a warm room can all bump a single value. Only the sustained pattern is meaningful.
It isn’t contraception. Temperature confirms after the fact; it cannot tell you a given day is “safe.”

The bottom line

Basal body temperature is a quiet, honest signal: a few tenths of a degree that say “ovulation happened.” It won’t predict your fertile days, but it confirms the one event your whole cycle is timed around — and that confirmation is what turns a fuzzy period estimate into a sharp one. If your watch already gathers it, letting your tracker read it is the easiest accuracy upgrade there is.

Just make sure that reading stays yours. In Dew, temperature is used to compute your prediction entirely on your iPhone — nothing is uploaded to be analysed elsewhere.