For more information about the menstrual cycle, check out the links below:

• Phases of the Menstrual Cycle
• What does temperature have to do with ovulation?

This blog is part of a series about ŌURA x Natural Cycles. To read the other posts, choose a link below:

• First Impressions of the Natural Cycles App
• Is ŌURA x Natural Cycles worth it?
• What happened in Natural Cycles when my ŌURA battery failed? — This was updated March 2023 because ŌURA released an firmware update for the ring that was intended to address the battery issue.

You can also read about my experience using the temperature sensor on the Apple Watch: Apple Watch Series 8 vs ŌURA x Natural Cycles vs Oral Temping: How do they compare for fertility awareness?

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I’ve been charting my menstrual cycle with the Symptothermal Method since 2015, determining fertile and non-fertile days manually. Considering my circumstances and PMDD, taking my temperature every morning was a small price to pay for 99.6% effective birth control that let me be who I am and live a much better life without synthetic hormones (1).

At the same time, I’d be lying if I said taking my temperature every morning didn’t get old after 7 years of it. Sure, it was a habit so I didn’t think too hard about it, but interpreting the temperatures was another story.

It sounds like a pain, but really, tracking my cycle with waking temperature was totally fine — until the last 18+ months when I was under immense stress and (I don’t use this word lightly) suffered from serious insomnia. My cycle became unpredictable as my body responded to the increased stress, and my temperatures and charts become increasingly difficult to read since waking temperature is unreliable without quality sleep (see more below about what can affect waking temperatures here).

Though my cycles have finally begun to re-balance, you can imagine how excited I was to receive the email from ŌURA about their partnership with Natural Cycles. Timing was serendipitous too: I was right in the middle of agonizing over 1) switching from Android to iPhone just to use the Apple Watch with its fancy new temperature sensors or 2) getting the Samsung Watch 5 and betting on future updates to utilize existing temperature sensors that don’t do anything yet.

ŌURA x Natural Cycles was a much easier choice than the two options I was considering, so I signed up immediately.

I’ve had two iterations of ŌURA ring over the last few years, so I’m familiar with them and trust their sensors. I had originally tried the gen 2 ŌURA ring to see if I could use their temperature deviation for cycle tracking, but other than my first cycle, the temperature readings were a little too noisy for me to rely on it exclusively, so I stuck with oral temping.

But now, with an updated ring, the right algorithm in Natural Cycles, and the FDA holding them accountable, I felt much more confident that the ŌURA ring could provide what I need. Theoretically, because it measure temperature throughout the night, it can capture true basal body temperature, thus giving me the confidence to confirm ovulation without other temperature devices.

For the record, I’m using ŌURA x Natural Cycles to streamline temperature measurements in conjunction with other fertility signs per the Symptothermal Method to determine my fertility. I would not rely solely on an app to determine if I should use protection on any given day.

Here are my honest thoughts and first impressions after using ŌURA x Natural Cycles for almost 3 cycles.

Notes: My experience in this post is based on a gen 3 ŌURA ring with the Natural Cycles and ŌURA apps for Android with a Pixel 6, an iPhone 12 Pro Max, and an iPhone 14 Pro. All screenshots are owned by their respective apps and were taken by the author.

Click a link below to jump to that section.

1. Fertility awareness methods and limitations
2. The ŌURA ring
3. Syncing the ŌURA ring’s data
4. Cost
5. Historical cycle data: Cycles 1–5
   1. An important note about comparing charts and temps
   2. Cycle 1: CD 20 vs CD 24
   3. Cycle 2: CD 20 vs CD 21
6. Real-time cycles with ŌURA x Natural Cycles
   1. Cycle 6 (partial)
   2. Cycle 7
   3. Cycle 8
7. Summary
8. Will ŌURA x Natural Cycles replace my oral thermometer?
9. Why I’m nervous but still switching
10. In closing
11. Resources and links
12. References

Fertility awareness methods and limitations

Fertility awareness methods (FAM), or fertility awareness-based methods (FABM), are a category of tracking the menstrual cycle that dates back decades. Modern, effective methods like the Symptothermal Method are often lumped in with outdated methods like the Calendar Method or the Standard Days Method.

Natural Cycles says they use previous cycle data and temperature to predict and confirm ovulation. But oral waking temperature cannot predict ovulation — it can only confirm ovulation (2). Read more about how temperature and ovulation are related here.

Natural Cycles is based on the Temperature Method and claims 98% effective with perfect use and 93% effective with typical use. The 2% that got pregnant with perfect use includes condoms breaking or the app giving an incorrect infertile or safe (green) day. If the condom breaks, it’s pretty obvious action needs to be taken right away, but users won’t know if the app gave an incorrect green day until it’s too late.

The Symptothermal Method is 99.6% effective because it uses multiple different body and fertility signs (temperature and cervical mucus, at minimum) to determine fertility each day, rather than trying to predict the future based on past cycles. It’s not perfect and is still subject to human error, misinterpreting charts, missing temperatures, and more. But because users are reading signs to determine fertility each day rather than relying on previous data to predict future data, it does account for the natural variability in each cycle. Every cycle is different, even for the same person, so trying to predict the future based on previous cycles is a gamble (3).

The ŌURA ring

Choosing the right size for the ŌURA ring can be tricky. They only come in whole sizes, so if you’re between sizes, it can be difficult to decide, especially since fit is so important for the sensors and it can’t be resized. When I bought my gen 3 ŌURA ring in 2021, I had a hard time choosing. ŌURA recommends using the index or middle fingers for the ring, but none of the available sizes would fit my index or middle finger comfortably, so I have to use the ŌURA ring on my ring finger.

The ŌURA ring is much thicker than a normal ring, so it may be irritating for some people to wear. Natural Cycles says ŌURA only has to be worn at night, so if the ring is uncomfortable, that’s an option. For me, it’s comfortable enough to wear all day and I like having the metrics from the full day.

The nice thing about the ŌURA ring is that you don’t have to think much about it. The company says anything that’s safe for your hands is safe for the ring, so you can shower or wash dishes with it. After your hands get wet, make sure you wipe the inside of the ring and your finger so your skin can dry a little better. I take it off when I shower and let my skin dry before I put it back on.

Another concern about the ŌURA ring is that the battery, like all batteries, won’t last forever. Using ŌURA x Natural Cycles long-term means committing to buying a $299 ring every few years when the battery dies (more about cost in the separate post above). That hurts a little less if you also use the sleep metrics too, but it’s still expensive. Plus, smaller batteries tend to have a shorter overall lifespan, so if you wear one of the smaller sizes like me, you may have to replace it sooner than someone who wears a larger size. I wear a 6, which is the smallest size, and my previous ŌURA ring lasted about 2 years before I started experiencing battery degradation. Battery lifespan depends on a variety of factors and may last more or less time depending on usage, though.

Syncing the ŌURA ring’s data

Each morning, I sync my ŌURA ring to the ŌURA app first, then I open Natural Cycles and wait for the apps to talk to each other. Sometimes it doesn’t sync right away, but you just have to give it a minute, and it comes through. The app has an option to force it to sync if you don’t want to wait.

Natural Cycles also has a button in the app to sync the ring if you don’t sync with the ŌURA app first. Tapping that button opens the ŌURA app, but I’m not sure that temperature syncing with Natural Cycles happens any faster with this method since I still had to wait in most cases (see the screenshot on the right).

For the first cycle, syncing was really smooth as long as I was patient. During the second and third cycles, syncing seemed to take longer, but I don’t know if it actually took longer or I just became impatient.

Once I’m in the fertile window, I know it’s not safe for unprotected sex until after ovulation. And I generally know if I’m in the fertile window or not since I’ve been tracking a long time and I also chart cervical mucus; I only really worry about syncing when I’m around ovulation.

Cost

Cost for this pairing is complicated topic, and I wanted to give it its own post: Is ŌURA x Natural Cycles worth it?

The bottom line is, to use ŌURA x Natural Cycles for ovulation confirmation, you need an ŌURA ring and a subscription to both Natural Cycles and ŌURA according to Natural Cycles.

Historical cycle data: Cycles 1–5

Since I had ŌURA temperatures for previous cycles, I was able to see them in ŌURA x Natural Cycles at work, retrospectively. This was exciting so I could immediately compare my charting to theirs instead of having to wait to ovulate in my current cycle.

The table below shows how Natural Cycles compared to my manual ovulation confirmation for my previous 5 cycles. Note: The table is numbered Cycles 1 through 5 because that’s how they are identified in Natural Cycles; in reality, Cycle 1 is Cycle 77 of my charting journey.

Cycle	Manual (Cycle Day)	Natural Cycles (Cycle Day)
1	20	24
2	20	21
3	18	18
4	18	18
5	21	21

Natural Cycles and manual charting agreed on ovulation day for 3 out of 5 cycles. Interestingly, Cycles 3 through 5 were actually a bit questionable for me when I was charting them on my own. I marked those ovulations based on cervical mucus and iffy temperature patterns, so it was nice to get confirmation from Natural Cycles that those days were correct.

Cycle 1 and 2 were a different story though. Let’s take a closer look, but first…

An important note about comparing charts and temps

To be clear, when I use two different methods of temperature collection at the same time for the same purpose, I am not comparing the actual temperatures. I’m not looking at each individual temperature and saying, “Well, my oral thermometer said 36.42° and ŌURA said 36.30°, therefore—”

Nope. That’s not the point.

First, thermometers are all different. I could take an oral temperature with two different thermometers at the same time on different sides of my mouth and still get different temperatures. As long as the temperatures are reasonably close to each other, it’s probably fine. If they have a major difference, I might change the battery or even get a new thermometer, but I never expect temperatures from different devices to match each other exactly.

Second, temperatures taken in different places of the body are going to be different because your body parts are generally not all the same temperatures.

Third, ŌURA isn’t measuring temperature just the one time like I do when I measure waking temperature. It’s measuring temperature every minute, leveraging insights from when I’m asleep because that’s when body temperature is most stable.

Finally, when I compare charts with two different temperature measurements, I don’t need an exact match of temperatures each day — I’m looking for the overall biphasic pattern that I usually see in my cycles. As long as the thermometer can accurately capture the pattern and temperature shift after ovulation, the actual temperatures don’t matter that much.

Cycle 1: CD 20 vs CD 24

Below is a screenshot of Cycle 1 from my manual tracking in Kindara. Here’s a quick guide on how to read the bars on the chart:

• Bars = volume and fertility of cervical mucus
• Pink = period
• Lower and darker orange = more fertile
• Gray = infertile
• No bar = I didn’t record any cervical mucus that day (not necessarily that I didn’t have any)

Also note: The cycle days between the two graphs below don’t lined up with each other.

Just at first glance, it’s pretty obvious that the manual temperatures are pretty noisy. Follicular phase temperatures are above the cover line and luteal phase temps dip below the cover line; in a perfect world, neither of these would happen. The temperatures from ŌURA in Natural Cycles are generally cleaner, with a clear biphasic pattern.

In Kindara for this cycle, I had 3 questionable temperatures as indicated by the gray dots in the Kindara graph. Those were unusually high, likely due to my waking up later than normal.

If I were strictly following the rules based on temperature, I would have marked CD 24 as the day of ovulation. But since cervical mucus had a straightforward pattern of wet to dry, I marked the temp shift on CD 20, even though the temp dipped below the cover line for a few days and rose again.

Natural Cycles indicated 4 more days as unsafe (red dots) than I did for this cycle, but that’s because I also track cervical mucus, whereas Natural Cycles does not use cervical mucus to determine fertility. Natural Cycles may have also been extra conservative because the algorithm presumably takes some time to get to know your body, and this was the first cycle, even if it’s historical data.

For this cycle, I would have considered the evening of CD 24 as a safe day for unprotected sex following the Symptothermal Method rules. If Natural Cycles is correct about ovulation being on CD 24, not CD 20 like I had indicated, I could have inadvertently gotten pregnant this cycle with my manual charting. Luckily, that didn’t happen, but the possibility is one of many reasons why I’d like something more reliable than taking a noisy oral temperature every morning.

Cycle 2: CD 20 vs CD 21

Ok, like I said, I have diligently temped nearly every morning for the last 7+ years, but during this cycle, we were dog sitting for my brother-in-law. His puppy jumped on my dresser, grabbed my thermometer, and chewed it up! Because of that, I didn’t have a thermometer to record temperatures for Cycle Days 16 through 19. Another point in favor of ŌURA since the dog couldn’t chew that — at least not while I’m wearing it.

Anyway so, on one hand, it is entirely possible that I would have marked a temp shift sooner, but with those missing temperatures, I had to mark it with the information I had. On the other hand, Cycle 2 had clear cervical mucus changes and temp shift (at least from that one temp on Cycle Day 20), so I was reasonably confident in ovulation at the time.

But Natural Cycles indicated ovulation on CD 21, which is one day after oral temperature did, not before. Also, ŌURA’s temps dropped for a couple days after and then jumped back up again. I’m curious how Natural Cycles determined ovulation in this case, but I wasn’t able to find information on the website about it.

Real-time cycles with ŌURA x Natural Cycles

I’m too impatient to wait for a new cycle to start a new temperature method, so I signed up with Natural Cycles when I got the email from ŌURA: Cycle Day 10. Many experts suggest you wait until the following cycle to start a new temperature measuring method, but I start immediately and don’t put as much weight into the partial cycle, using extra precaution when deciding non-fertile days.

Cycle 6 (partial)

In this cycle, I was interested to see how Natural Cycles will handle my first real-time cycle since it has temperature data from ŌURA for my previous 5 cycles. Customer service at Natural Cycles confirmed via email that the algorithm would know my cycles faster because of those previous cycles.

During the first week with ŌURA x Natural Cycles, I got my covid booster shot. The following morning, my oral waking temperature was unusually high and so was the deviation in ŌURA. I marked the oral temp as questionable in Kindara manually and opened Natural Cycles to do the same.

I was pleasantly surprised to be greeted with this message explaining that the algorithm identified the high temperature as anomalous and automatically excluded it from calculations. The message also appeared in the Messages section of the app, which was handy since I wanted to read it again later.

On the Today screen, the app still showed the recorded temperature, but it’s crossed out and darker than normal. Underneath the temperature is a message in red that says “Excluded by the algorithm,” so it’s very clear that the temperature is an outlier. I don’t know if it would have been able to identify this outlying temperature if it didn’t have data from the previous 5 cycles, so your mileage may vary on your first cycle.

On the graph, the outlying temperature does not appear at all, which makes it look like the data is simply missing. I would have preferred if they showed the temperature in another color and not connected to the other temperatures (like in Kindara) or some other visual cue to show that the temperature was taken but not included.

Later in the cycle when I saw the missing temperature on the graph (the graph below has no dot between CD 13 and 15), I didn’t make the connection right away that it was the day of the outlying temperature, so I was worried something was broken. I eventually figured it out, but the app could have done a better job here.

When I passed the predicted ovulation day (Thursday), the egg symbol changed to a clock and said “Waiting to confirm” underneath it.

See below for my Kindara (top) and Natural Cycles (bottom) graphs. Note: The cycle days between the two apps don’t line up with each other.

Ovulation matched up this time on CD 23! Manual temps once again dipped below the cover line during the luteal phase, but cervical mucus was drying up, so I still marked it as CD 23.

Cycle 7

My second cycle comparing manual and ŌURA temperatures was trickier. Manual temperatures were all over the place! ŌURA temperatures were slightly better.

This cycle, I also used luteinizing hormone tests (LH strips) to add to Natural Cycles. It’s not required but supposedly helps the algorithm. You can see the results of the LH tests in the blue squares above the Natural Cycles temp graph on CD 17 and 18. Natural Cycles conveniently tells users when to take these tests, and the app has a feature in beta to even read the test for you.

Regardless of the crazy temperatures with the manual thermometer, ŌURA and I both agreed ovulation occurred on CD 20.

Cycle 8

This time, Natural Cycles confirmed ovulation on CD 21 whereas I marked ovulation as CD 22. I also had 2 days of unusually high temps in Natural Cycles due to illness (see the missing temps for CD 18 and CD 19 in the graphs below), though manual temperature only caught the high temp on the day I had an actual fever.

Natural Cycles gave me the green light on CD 24 because ovulation was confirmed for CD 21. This was only a day off from what I had confirmed using cervical mucus. Temperatures were pretty biphasic and my mucus had dried up, so I felt pretty good about it overall.

But the real weirdness in this cycle didn’t happen until late into the luteal phase.

On CD 36, my temperature dropped, which is normal for me when I’m about to start my period. Natural Cycles marked the day as a fertile red day, and moved ovulation status back to waiting to confirm for days CD 21 to CD 23.

CD 21 to CD 23 were all red days even when ovulation had been confirmed before, so following the Natural Cycles rules would have still held up, but it’s incredibly alarming that the app can change its mind on ovulation like that. What if one of those days hadn’t been a red day previously?

With my manual charting, I had marked ovulation on CD 22, a day later than Natural Cycles. Following the Symptothermal Method, I waited for CD 26 to consider myself infertile (see Kindara chart below) anyway. But a Natural Cycles user who doesn’t know the Symptothermal Method or doesn’t chart cervical mucus may not be so conservative.

I haven’t read any studies or books that indicate you can return to a fertile state during the luteal phase, as long as you’ve confirmed ovulation. Why would a temperature 15 days into the luteal phase change my fertile status? This could be a case of the app being extremely conservative, but the fact that a confirmed ovulation can change back to unconfirmed status makes me really nervous. It’s not really confirmed if it can still change. If ovulation only changes by a day in either direction, that doesn’t make today not safe since it was still over 10 days ago, so what’s happening here?

It’s possible the app just changed the status to a red day because that’s automatic if ovulation status is waiting to confirm. But still, how could a temperature so far from ovulation change the date of ovulation?

The good news is, Natural Cycles did not change any previously green days to red (not sure if this is even a thing that happens), so that’s a relief.

I ended up getting my period later that same day (which I totally expected), so Natural Cycles switched it to a green day as soon as I recorded a period, and ovulation also updated to confirmed status once again. But it sure caused a panic in the morning.

Even though it worked itself out, I emailed Natural Cycles for clarification about what would cause an ovulation status to change and if it was related to the low temperature in the luteal phase. Their initial response was that ovulation was confirmed and that follicular phase temperatures are always low. Since they were only seeing my account in its current state, they didn’t understand my question, so I responded with a more detailed explanation including screenshots of the confirmed and subsequently unconfirmed ovulation. I will update this when I hear back from them. Hopefully they have a more satisfactory response this time.

*Update* Natural Cycles responded and said the temperature was low for the luteal phase, so the app changed the day to red out of precaution.

When your temperature has risen enough to confirm ovulation, the most likely ovulation day is calculated from the temperature curve. Please note that the algorithm may move the confirmed ovulation day to another day should your newly added data support this. […]

You were switched back to red days on CD36 (now CD1) because of the lower temperatures that you have added and the fact that you had not yet logged your period. These temperatures are slightly too low for the luteal phase, and thus the algorithm, out of precaution, has switched you back to red days. Once you logged your period, the algorithm was able to confirm ovulation again, since a new cycle started and the day turned green. 

Natural Cycles customer service

They didn’t explain why a temperature 15 days into the luteal phase would change the ovulation status, but I kinda gave up trying since the answer always seems to be “because the algorithm.”

Again, the oral temperatures were really noisy for this cycle. Normally in my luteal phase, I wouldn’t get dips like on CD 6 and 7, although that was probably due to waking up earlier than normal, and I probably could’ve marked those as questionable. In the Natural Cycles graph, you can see a few squares at the top indicating when I used LH strips, I was sick, plus recorded 2 other things (likely moods or symptoms).

Summary

Whew! This was a lot of information, so here’s the summary of the pros and cons of using ŌURA x Natural Cycles.

Pros	Cons
✅ Hormone-free 98% effective with perfect use ✅ Simple temperature syncing ✅ Use with Apple or Android devices ✅ Effortless, unobtrusive temperature measurement ✅ Can wear ŌURA only at night if desired ✅ Ovulation prediction and confirmation ✅ Clear yes or no for daily fertility ✅ Temperatures don’t seem as noisy as oral	❌ Only 93% effective with typical use ❌ Requires two apps to work properly ❌ Two subscription fees add up ❌ ŌURA ring device is expensive ❌ ŌURA sizing may be too limited for some users ❌ ŌURA battery cannot be replaced once spent ❌ Need another app to track detailed or custom symptoms if desired

It’s so hard to know whether or a not a device is going to work for you, especially with something as unique as your menstrual cycle and as important as avoiding pregnancy. I’ve explained my experience in great detail, but as always, your mileage may vary and the choice is ultimately yours to make.

I’m sure this will be annoying, but I can’t recommend for or against ŌURA x Natural Cycles because it’s such a personal thing. Here are some things that might help you decide for yourself.

ŌURA x Natural Cycles may work for you if you’re:

• An experienced fertility awareness method user looking to replace your daily oral temping and confirm ovulation.
• Interested in tracking sleep metrics too.
• Not worried about budget.
• Already own and subscribe to ŌURA products.
• Not an experience fertility awareness method user but are willing to accept 2-7% risk of an unwanted pregnancy.
• Willing to abstain from sex or use other contraceptives methods during your fertile days.

Switching to ŌURA x Natural Cycles is probably not worth the cost if all of the following are true for you:

• You have experience with cycle charting.
• You’re happy with your current temping method.
• You don’t generally need help confirming ovulation.

Will ŌURA x Natural Cycles replace my oral thermometer?

When I first started using ŌURA with Natural Cycles, I was (to borrow a phrase from Adam Savage) cautiously optimistic that this would work well for me. Now I’ve been using ŌURA x Natural Cycles for 3 months, but I was hoping to be more confident after 3 cycles.

I’m reasonably confident in ŌURA’s sensors and data, so I’m mostly willing to count on them to make my life a little easier.

So the answer is yes, but…

Let’s be clear: I am not nor would I blindly trust an app to determine my fertility status. I said this earlier, but that was many paragraphs ago and I want to say it again: I’m using ŌURA x Natural Cycles to streamline temperature measurements in conjunction with other fertility signs following the Symptothermal Method to determine my fertility.

Why I’m nervous but still switching

I’m nervous about switching for a few reasons. For example, I’m slightly suspicious of how tidy the data is. It’s not perfect, but it’s much more clearly biphasic than my oral temperatures. On one hand, that’s exactly what I want and would expect from continuous measurement. On the other hand, such tidy data is unfamiliar to me and, in a way, doesn’t seem like it’s real.

To help me fully think through this uneasiness, I took another look at the comparison chart with the 5 historical cycles plus the 3 cycles I charted live with Natural Cycles.

Cycle	Manual (Cycle Day)	Natural Cycles (Cycle Day)
1	20	24
2	20	21
3	18	18
4	18	18
5	21	21
6	23	23
7	20	20
8	22	21

Out of 8 cycles, 37.5% had a different ovulation day than I did with manual measurement, Cycle 1, Cycle 2, and Cycle 8. (I recognize that 8 cycles isn’t necessarily statistically significant, but it’s all I’ve got right now.) Of those 3 that had differences, only Cycle 1 had more than 1 day difference, and I can easily see why Natural Cycles confirmed that day for ovulation instead of the day that I chose.

Something to keep in mind about this comparison is that I don’t know which of these is when ovulation actually occurred — the only way to know that would have been to have an ultrasound that day.

Theoretically, Natural Cycles would be more accurate because ŌURA measures temperature throughout the night and isn’t affected by the same factors as oral thermometers. But with the Symptothermal Method, I mark ovulation based on temperatures and cervical mucus, which is a more complete picture of fertility than temperature alone (4). So that makes me suspect manual may be more accurate.

Also, Natural Cycles suddenly changing to unconfirmed ovulation and a red day 15 days into the luteal phase was alarming.

In any case, I feel comfortable using ŌURA x Natural Cycles to replace my temperature measurement only, but I will continue to monitor cervical mucus, cervical position, and sometimes luteinizing hormone and progesterone, rather than relying solely on the Natural Cycles app.

In closing

So yes, ŌURA x Natural Cycles is replacing my daily morning temperature measurement, but no, Natural Cycles is not replacing the Symptothermal Method of charting. In fact, cervical mucus has been shown to be a better prediction of fertility and pregnancy probability (5), so I’ll definitely continue to chart cervical mucus and weigh that heavily when determining my fertility status for the day.

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Resources and links

This content may contain affiliate links for products I use and believe in. If you subscribe or make a purchase after clicking one of these links, I’ll earn some money at no extra cost to you. I deeply appreciate your support so I can keep doing what I love — providing helpful content to readers like you! Thank you!

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References

1. Centers for Disease Control and Prevention. (2014, April). Providing Quality Family Planning Services Recommendations of CDC and the U.S. Office of Population Affairs, Appendix D: Contraceptive Effectiveness.
2. Shea, A. (2021). Why basal body temperature (BBT) isn’t the most accurate ovulation predictor. https://ro.co/health-guide/basal-temperature/
3. Fehring, R. J., Schneider, M., & Raviele, K. (2006). Variability in the phases of the menstrual cycle. Journal of Obstetric, Gynecologic, & Neonatal Nursing, 35(3), 376-384.
4. Weschler, T. (2015). Taking Charge of Your Fertility: The definitive Guide to Natural Birth Control, Pregnancy Achievement, and Reproductive Health.
5. Bigelow, J. L., Dunson, D. B., Stanford, J. B., Ecochard, R., Gnoth, C., & Colombo, B. (2004). Mucus observations in the fertile window: a better predictor of conception than timing of intercourse. Human Reproduction, 19(4), 889-892.