For as long as there have been humans, there have been people trying to plan when they conceive. Sometimes they want to use information or tools to avoid conception, for health, financial, or personal reasons. Other times they want to use the same information to increase their chances at conception because they've decided that now is the appropriate time to bring a new life into the world.
If you look throughout history, you can find everything from using specific kinds of teas to heavy metals to physical poking and prodding to manipulate, control, and otherwise gauge fertility. Some of those methods are slightly rooted in fact, while others are mere superstition.
You would think that in today's modern, enlightened age, we'd be more immune to nonsense superstition and would only use methods based on some reputable, repeatable science. And yet, technology itself doesn't guarantee viability. There are plenty of instances of technology being used to sell snake oil and superstition for a modern age.
Enter the fertility monitor.
What is a Fertility Monitor?
The first question you may have is "what even is a fertility monitor?" Basically, a fertility monitor is an electronic device or app that tracks data about you and guesses what your fertility level is. They work using a handful of different methods.
There are generally three kinds of fertility monitors, and we'll talk about all three of them in this post. 
They are:
- Apps. Fertility monitoring apps are generally calendar programs that you can use to track ovulation and estimate fertile days based on general human biology. These tend to be free or very cheap, and can also be found on websites or done manually with a calendar.
- Biometric trackers. These devices are similar to a FitBit fitness tracker but are aimed at monitoring vital signs like body temperature to estimate fertile times. These tend to be a little more expensive and might have additional features like sleep quality, heart rate tracking, and other biometric data.
- Full monitors. These devices check everything from electrical resistance in vaginal fluids to hormone levels in urine to estimate fertility. These tend to be the most expensive, and some of them may require consumable kits or individual tests you can only use once, so they have an ongoing fee, at least until you establish your baseline cycle data.
As you might expect, different kinds of monitors work in different ways and have different levels of efficacy.
How a Fertility Monitor Works
A woman's body typically cycles between high fertility and low fertility on a near-monthly basis. They are based on some observable changes in the body, some of which are obvious physical changes and some of which require a more comprehensive test to check.
Apps are generally little more than calendars that help you track your period, which correlates to fertile times when you're ovulating. Since your smartphone isn't going to be able to test your hormones or electrical conductivity, it won't be doing anything significantly advanced. You can replicate an app's functionality with a calendar and some dedicated tracking, but an app basically removes the busywork from the equation.
For biometric trackers, the usual sign they track is body temperature. Your "basal body temperature" is your body temperature when you're fully and completely at rest. Thus, taking that temperature is best done first thing in the morning, before you so much as move to get out of bed. No eating, no drinking, no smoking, nothing; just take your temperature before you get out of bed each morning, chart it, and check for changes on a monthly cycle.
Typically, your basal body temperature will increase slightly when you're ovulating. Your most fertile days will be 1-3 days before this temperature increase, so by charting the number of days between increases, you can estimate the days you will be fertile.
A biometric tracker device will usually be designed to monitor your overall body temperature throughout the day, estimating your basal temperature by correlating it with data about sleep, resting heart rate, and other activities. The viability of this kind of device depends on how often you wear it and how consistently it can measure your temperature down to tenths of a degree.
Full fertility monitors work in a few different ways. They can have calendars and temperature tracking built into them, but they can also test other indicators of fertility as well.
One of the more common tests is the urine test. A urine test is similar to a pregnancy test; it just checks for hormone levels in your urine when you use it. They often check for hormones like progesterone metabolites and luteinizing hormones, which spike when you ovulate. By taking this kind of test each day, you can estimate when you're ovulating, and again, correlate this with the length of a cycle and guess when you're going to be most fertile each cycle.
Another kind of test is the electrical conductivity test. There are two variations of this; saliva and vaginal mucous. The idea is that electrical conductivity will change in both substances as part of the overall bodily changes that happen during ovulation. Electrical conductivity has been observed to decrease before ovulation and increase afterward in sows, and have a slight correlation with luteinizing hormone spikes in saliva, though how reliable those results are is still undecided.
How to Use a Fertility Monitor
The specific instructions on how to use a fertility monitor vary as widely as the number of monitors on the market.
Apps are, in general, easy to use. You simply chart the information they ask for, like the first day of your period, basal body temperature, or other bodily signs you can check without needing special tools. They then use their calendar functionality to give you an idea of when you're ovulating, when you're fertile, and when you're least fertile. This data will be somewhat accurate, though it tends to be generic to women across the board and won't necessarily take specific medical conditions or other individual factors into consideration.
Fitness monitors and biometric tracks are essentially calendar apps with some simple biometric tracking added on top. They, too, can be accurate, but are only as accurate as the data they can acquire. The cheaper kinds of biometric monitors aren't very precise or accurate, and so they might have a harder time actually tracking your cycles. More expensive and more detailed devices might be more accurate.
Full fertility monitors tend to be most accurate, though more intensive to use. You will have to actually perform mucous/urine/saliva tests on a regular (possibly daily) basis, until you establish enough information to figure out your bodily baselines, your spikes, and your cycles. Once you have that information, you can then, again, correlate this information with a calendar and use it to predict when you're going to be fertile.
Do Fertility Monitors Work?
The answer to this is quite tricky. In theory, fertility monitors should work. After all, your body does experience tangible changes before, during, and after ovulation, and those changes can be monitored and tracked.
Whether or not a fertility monitor "works" depends on what you want to use it to accomplish.
If you want to use a fertility monitor to help avoid conception, as a form of birth control, their utility is potentially questionable. No method of birth control short of sterilization is 100% effective, so tracking cycles is best done as a supplement to go alongside other methods of birth control.
Conversely, if you're trying to conceive, a fertility monitor has the potential to help. We say "the potential" here simply because they are only as effective as their tests.
An app-based calendar tracker can help, but again, they use aggregate data for the typical woman's cycle and are unlikely to adapt to your specific situation. They can provide you an idea of when your fertile window is, but it's simply a guess; without actual, tangible data, they can't do more than guess.
A biometric monitor, which is usually just monitoring your basal body temperature, can potentially be effective if it's sensitive and accurate enough. However, there are a lot of different ways this reading can be thrown off. A minor illness – or a major disease – can throw off readings of your body temperature. The monitor itself might not be sensitive enough to give you really accurate data. It might also be easy to use improperly and thus fail to get the really accurate data you need.
Even after all of that, you still are back to simple calendar charting, just with a single data point – temperature – to use to adjust that tracking. It's more effective than simple cycle guesswork, but that's not necessarily saying a lot.
Full monitors with several tests, like hormone tests and conductivity tests, tend to be more accurate. There's a major caveat though; they have to be effective, accurate tests. Electrical conductivity in saliva and vaginal liquid can be variable based on a number of different factors, including diet, exercise, time of day, and more. It's not always a reliable indicator. Plus, these tests can be difficult to use properly, and uncomfortable, so taking them every day as necessary to gather enough data might be hard.
Hormone tests are the most accurate, but they're also the most expensive, primarily because they tend to be disposable. They also rely on accurate testing of hormone levels, which again depends on trusting the manufacturer to make accurate tests. These days, that's not always guaranteed. Even big-name companies might not actually be reputable just because they're big.
Can a Fertility Monitor Help You Conceive?
Yes, possibly, a fertility monitor may work for you.
Here's the thing: you can't treat a fertility monitor as a 100% accurate reading of your body. They are by necessity going off of standardized data, and as such, they can only chart your cycles. You still need to be able to measure those cycles accurately and make a judgment based on them.
To the extent that charting data works, then yes, a fertility monitor can work for you. If you use it to estimate your fertile period, attempt to conceive during that period, and successfully conceive, then the monitor worked for you. It is, after all, a binary outcome; either you conceive or you don't. If using a monitor helps you conceive, then it works.
Using a Fertility Monitor as an Intermediate Step
Women who want to conceive are often worried when it doesn't work right away, and the thought of having fertility issues can be stressful and nightmare-inducing. Using a fertility monitor can provide some peace of mind.
Think about it this way: a fertility monitor will have a reasonable idea of what a baseline cycle should look like. If you take data on an ongoing basis for a month or two, and your cycles match what they should look like, you can have a better idea that there's nothing wrong with your body and that conception simply takes time. Conversely, if your data doesn't match up or shows some level of abnormality, you can use that as a reason to visit a doctor to discuss your fertility.
A doctor will of course be able to perform more detailed tests, including everything from more rigorous hormone tests to blood panels to examinations of the uterus and ovaries to look for potential problems. You can then identify issues like PCOS, Endometriosis, or other fertility issues.
Essentially, you can use a fertility monitor as a step between simply going for it, charting, and visiting a doctor. Consider it a slow escalation. If conception isn't working, try using a monitor. If the monitor isn't helping after several months, consider visiting a doctor.
There's nothing wrong with the theory behind fertility monitors. As long as the tests they perform and the data they monitor is accurate, and you don't have any underlying health issues, there's no reason they wouldn't be able to help you conceive.
