If you’re new to fertility awareness, check out Phases of the Menstrual Cycle and What does temperature have to do with ovulation? for some more basic information.

When the Apple Watch Series 8 came out with advanced temperature tracking and ovulation confirmation, I knew I had to try it. In fact, I switched from Android to Apple just to keep using the watch!

Unfortunately, Apple Watch’s temperature tracking didn’t work out like I wanted, though the watch is still worth having. Here’s my experience with the Apple Watch using an iPhone 12 Pro Max and an iPhone 14 Pro.

One important note if you’re thinking of using the temperature sensor: The Apple Watch doesn’t record temperature unless the watch is in sleep mode.

All screenshots are property of their respective owners and taken by the author.

Click a link below to jump to that section:

1. Important note about temperature devices
2. Comparison table
3. Cycle 1
4. Cycle 2
5. Cycle 3
6. Would I use Apple Watch to replace oral temperature?
7. In closing

Important note about temperature devices

Whenever compare different methods of temperature collection at the same time for the same purpose, I’m not comparing the literal temperature values. I don’t review individual temperatures and think, “My oral thermometer said 36.32°C and Apple Watch said 36.12°C, therefore—”

Nope.

First, thermometers are all different. I have taken a temperature by mouth with two thermometers at the same time and still got different temperatures. What I’ve realized is that as long as the temperatures from the different devices are reasonably close to each other, it’s probably fine. If they have a major difference, I might be more concerned, but I never expect temperatures from different devices to match each other exactly.

Second, temperatures taken from different areas of the body are going to be different because different body parts are generally different temperatures.

Third, the Apple Watch isn’t measuring temperature just the one time like I do when I measure waking temperature. It’s measuring skin temperature at the wrist throughout the night.

Last, when I compare charts with two temperature devices, I don’t need an exact match of temperatures each day — I’m looking for the biphasic pattern that I usually see in my cycles. As long as the thermometer captures the temperature shift after ovulation, the actual temperatures don’t matter that much. That’s why the table below compares the cycle day (CD) that ovulation was confirm by each method, not the individual temperatures. I do provide the complete graphs below so you can see the overall patterns, though.

Comparison table

So far, I’ve used the Apple Watch, ŌURA x Natural Cycles, and oral temperature for 3 complete cycles. Below is a chart comparing what cycle day each method indicated ovulation occurred.

Cycle	Manual (CD)	Natural Cycles (CD)	Apple Watch Series 8 (CD)
1	20	20	19
2	22	21	20
3	17	17	N/A

Ovulation confirmation via oral temperature and cervical mucus, ŌURA x Natural Cycles, and Apple Watch Series 8 is pretty comparable — when I get a confirmation. The first couple cycles weren’t more than 2 days different from each other, which is not bad, but the third cycle didn’t get confirmation from Apple.

At first I marked CD 19 as the shift with manual charting, but I decided to change it to CD 17 because of cervical mucus and the temperatures were a bit ambiguous from poor sleep. Natural Cycles confirmed CD 17, which was another reason I decided to switch from CD 19 to CD 17. But Apple Watch wasn’t able to confirm ovulation for that third cycle at all.

The graphs below show the temperatures from ŌURA x Natural Cycles, oral measurement, and Apple Watch Series 8.

Cycle 1

Natural Cycles confirmed ovulation on CD 20, indicated by the pink bar and the egg icon. The Apple Watch confirmed ovulation for this cycle, indicated by the vertical purple bar. Oral temperatures are labeled BBT in the graph above the Apple Watch temperatures. I confirmed ovulation manually with oral temperature and cervical mucus patterns on CD 20, but that’s not shown on the graph.

Natural Cycles has a pretty nice biphasic temperature graph, with most pre-ovulatory temperatures below the baseline and post-ovulatory temperatures mostly above the baseline. But both oral temperature and the Apple Watch’s wrist temperatures are pretty noisy.

Cycle 2

Natural Cycles temperatures are again nice and biphasic. Oral waking temperatures were a little less noisy, which results in a more biphasic pattern, but the Apple Watch’s graph continues to be above and below the baseline without any discernible pattern.

Interestingly, Apple Watch did not confirm ovulation for this cycle until after I recorded my next period.

I confirmed ovulation with manual temperatures and cervical mucus a day later in this cycle than Natural Cycles and 2 days later than Apple Watch. I waited to mark ovulation with manual temperatures because cervical mucus didn’t dry up until CD 22.

But the Apple Watch graph does get a nice 5 consecutive days of temps above the baseline from CD 21 to CD 25 before it dips below the baseline again — that’s enough to confirm ovulation with oral temps.

This cycle gave me hope that the Apple Watch could work for fertility awareness. But that hope was dashed just 30 days later.

Cycle 3

During the third cycle, I had an unexpected snafu: The battery in my ŌURA ring failed. It still worked, but I missed 2 nights of temperatures (not consecutive) because of the battery issue.

I accidentally slept without putting my watch back on, so Apple Health is also missing a temperature. Luckily that happened after ovulation, but Apple Health was not able to confirm ovulation for this cycle. The missing temperature was on CD 23, so that shouldn’t have affected the ovulation confirmation, but I don’t know enough about how they confirm ovulation to know if that’s why ovulation wasn’t confirmed.

Apple Health clearly says when it can’t estimate ovulation, which I appreciate, but it doesn’t say why not. The Learn More link in the notification just talks about how environmental factors or other things may contribute to this happening. It also says the cycle may have been anovulatory, or that ovulation may have occurred without an obvious temperature shift. These are all possible, but nothing is specific to how Apple confirms ovulation.

Either way, the Apple Watch data is noisier than waking temperature, which I’m guessing is why it couldn’t confirm ovulation. I wasn’t super confident in my manual charting this time either, but cervical mucus had a clear pattern, and Natural Cycles had a good guess, which it confirmed after I started my next cycle.

Would I use Apple Watch to replace oral temperature?

Originally, I wanted to try the Apple Watch to replace taking my oral temperature every morning. But after just these 3 cycles, I’d say no. The temperatures don’t have a clear biphasic pattern, so if I didn’t take my own temperature, I’d be completely reliant on Apple’s algorithm to confirm ovulation. Since Apple Health sometimes doesn’t confirm ovulation at all or waits until after I start my next cycle, I can’t count on it for fertility awareness yet.

The fact that Apple Watch waits until after I start my next cycle also makes me wonder if they’re simply calculating how long my luteal phase was and estimating based on that, which is again, less useful than manual charting.

It is possible that the watch needs more time to get to know my temperatures, so I’ll continue wearing it to sleep, but I won’t rely on the Apple Watch Series 8 for my fertility awareness method.

In closing

Wrist temperature seems just as noisy as oral temperature, if not more so. This is the first Apple Watch with the temperature sensor, and it has the additional sensor to check the outside temperature, but I’m not sure how well that works. It sounds good in theory, but I often sleep with my hand under the blanket, and I’m almost always in sleeves long enough to cover the watch, so I’m not sure if the outside sensor is doing any good.

In any case, until Apple Watch improves this feature, I’ll stick with oral temperatures. Though I’m currently testing TempDrop, so hopefully that works out. Stay tuned!

Check out the series below for details about my experience with ŌURA x Natural Cycles:

• First Impressions of the Natural Cycles App
• Is ŌURA x Natural Cycles worth it?
• ŌURA x Natural Cycles Review: Can it replace fertility awareness charting?
• What happened in Natural Cycles when my ŌURA battery failed? — This has a quick update from March 2023 since ŌURA released a firmware fix for the battery issue.