Can an app replace your birth control? For most people, the answer is no.

No birth control is for everyone, and that’s especially true for apps. The Food and Drug Administration just approved the first software application for contraceptive use—Natural Cycles—and it’s raising a lot of eyebrows (Editor’s note: Bonnier Ventures, the investing arm of Bonnier Group, which owns Popular Science, has financial investments in Natural Cycles).

The method that the app is based on has been in use for hundreds of years, at least. Women have tracked their periods, looked at their cervical mucus, and taken their body temperatures in order to predict their ovulation for centuries, both to get pregnant and to avoid pregnancy. But it isn’t the most effective method. The Centers for Disease Control puts fertility tracking at only 76 percent efficacy. So how can Natural Cycles claim a 93 percent efficacy rate? And if it’s so great, should you be using it?

While the new tech might bolster the existing routines of some potential users, it’s definitely not for everyone. Here’s everything you need to know if you’re thinking about giving it a go:

The app works kind of like fertility tracking, but with more data

Fertility tracking traditionally relied on a variety of factors, from period logging to mucous thickening, but Natural Cycles mainly works using your daily basal temperature. That’s essentially your body’s resting temperature, which is most accurate first thing in the morning when you’ve been sleeping for many hours. Ovulation causes a slight uptick in basal body temperature, which is why many tracking methods use it to try to predict fertility.

To use the app, you’re supposed to take your oral temperature right when you wake up using the provided thermometer. Logging this each day allows the app to learn the patterns of your specific ovulation cycle. That means when you first start, you won’t be able to take any hormonal birth control, which would muck up the data. But you can’t have unprotected sex, either, because the app doesn’t have enough data yet to predict ovulation. That means every day will be red, the indicator for either not having sex or having sex with a condom (side note: the risk of pregnancy with a condom given typical use is 18 percent). Eventually, you’ll get down to less than a week of red days a month, with green days in between to indicate that unprotected sex should be safe (assuming you’re not worried about sexually transmitted infections, which an app can’t protect against).

This method isn’t so different from tracking your own ovulation by hand with a calendar, but it does take the calculation off your mind. The app does the work for you, and you just have to remember to take your temperature at least five times per week.

Natural Cycles works best for people with very stable lives

Though fertility tracking gets a bad rap, it is actually possible to successfully avoid pregnancy by abstaining from sex (or using protection) when you’re ovulating. This is simply because you can only get pregnant when there’s an available egg to greet incoming sperm. If there were a device that could predict your ovulation with 100 percent accuracy, that device would give you spectacular contraceptive results (assuming you actually followed its advice).

But that device doesn’t exist. There are strips that can test your urine for luteinizing hormone, which your body releases when ovulating, and these can be a great addition to tracking your cycle (Natural Cycles can incorporate that information in its algorithm). But the strips aren’t perfect. You have to use them first thing in the morning, and they can be inaccurate if you test too early or too late in your cycle. And (somewhat confusingly) you’re not the most fertile right as you ovulate. Your window is four to five days before ovulation, whereas the test strips signal that ovulation is roughly 24 hours away. That means a negative indicator on the strip isn’t actually a good go-ahead to have unprotected sex.

Menstrual cycles aren’t precise machines: they’re complex beasts involving multiple types of hormones that themselves can be affected by any number of outside influences, from stress to body fat percentage.

Natural Cycles acknowledges these influences and works around them in two main ways. The first is to be overly cautious. Most women should get four or five red days per month, but if there’s uncertainty the app errs on the side of giving you a red day instead of a green one.

The second is that the user manual gives you a list of scenarios where you may not want to log your temperature on a particular day, or should at least tick off the “deviation” option to indicate that it may vary from your norm. That’s because lots of factors can affect your basal temperature that have nothing to do with menstrual cycles. These include not sleeping well the night before, having an infection, having a hangover, taking certain antibiotics, feeling stressed, and traveling. The website also notes that your temperature can be off if you had more than two alcoholic drinks the night before or if you got up in the middle of the night to use the bathroom or get a glass of water. It can even be off if you’ve slept a different number of hours than usual, which is why the company recommends against logging your temperature over the weekend.

If you’re a person who experiences these kinds of disruptions regularly, the app may be less accurate for you. Maybe you sometimes work night shifts. Maybe you have a stressful job. Maybe you’re a student and sleep irregular hours. It’s not that you can’t use the app, but you should know the algorithm won’t work as advertised for you.

That 93 percent efficacy rating might not even be accurate

Natural Cycles has advertised its 93 percent efficacy a lot, in part because that number is slightly higher than the one for birth control pills (that’s 91 percent). There are a few things we should note about those stats, though.

First off, we should note that both of those efficacy numbers assume what’s called “typical use.” That means they take into account a typical amount of human error. For example, if you took your pill at exactly the same time every single night, hormonal birth control pills would be 99.7 percent effective. That’s perfect use, and it means for every 100 people taking the pill for a year, there would be an average of 0.3 pregnancies. But humans are fallible. They go to parties and travel between time zones and just plain forget, so “typical use” offers a more realistic view of contraceptive efficacy. Pills can be 99.7 percent effective, but are just 91 percent effective the way most people take them.

Contraceptives that rely on human memory tend to have a big gap between typical and perfect use. Methods like IUDs (such as ParaGard and Mirena) and hormonal implants stay in place all the time, so they have ideal and typical use failure rates that basically match. Copper IUDs fail 0.6 percent of the time, hormonal ones fail 0.2 percent of the time, and implants just 0.05 percent of the time. Even sterilization doesn’t match that: Female sterilization comes in at a 0.5 percent failure rate, while male comes in at 0.1 percent.

In other words: there are many other birth control options that still beat Natural Cycle’s supposed 93 percent efficacy—and if you don’t have perfect use with the pill, good luck managing it with an app. If you have trouble remembering to take a pill at the same time every day, taking your temperature at the same time every day probably won’t be easier—and if you’re not likely to be consistent with your temperature readings, Natural Cycles is also probably not going to work well for you.

But if that 93 percent is typical use, doesn’t it mean it holds true for everyone—even folks who are forgetful? Probably not.

The number comes from a study that’s been criticized by some researchers. One health expert told The Guardian that it was “inappropriate and misleading.” That’s because the study recruited participants by asking existing users to volunteer, which could have led to a skewed population. People who accidentally got pregnant on the app or for whom the algorithm didn’t work on (or who simply realized they were doing a poor job of keeping up with the data collection) wouldn’t be likely to keep using the app, or to volunteer for a study in which they’d have to keep using it. So the design of Natural Cycles’ study makes it likely that most people involved in the trial would avoid pregnancy. Again, traditional fertility tracking only has 76 percent efficacy, and while the app may well be more accurate than manual calculations, it may also not be as high for the average ovulating human off the street as Natural Cycles is claiming.

It’s also worth mentioning that even if the pill really is less effective, if you take it every day, it protects you every day. Most birth control pills will continue to offer some protection if you miss a day, especially if you take the skipped dose as soon as you remember. IUDs, injections, and implants all offer 24/7 protection. You have to know that they might not work for you, but you don’t have to change your behavior from one day to the next in order to maintain that protection.

If you use a fertility tracking app and accidentally have sex on a ‘red’ day, well, you just gave yourself a decent shot at getting pregnant. Of course, many users can choose to use emergency contraception as a backup, and everyone relying on the app should consider using condoms no matter what day of the month it is. But if you’re switching from the pill to a fertility tracking app, it’s important that you really get into your noggin just how different this new form of protection is.

The app could be great if you’re not using hormonal birth control, or if you’re hoping to get pregnant soon

As is the case for all birth control methods, there is an ideal Natural Cycles user. Some people may not like the idea of adding hormones to their bodies or may have trouble tolerating hormonal forms of birth control. Many opt for the non-hormonal copper IUD, but this can cause painful cramping and heavy periods for some—so you may prefer the tracking method. If you’re not otherwise using a form of birth control besides condoms (please, please use condoms), adding Natural Cycles could significantly decrease your chances of getting pregnant. Male condoms fail 18 percent of the time, so doubling up could be worthwhile. Natural Cycles can tell you which days it’s relatively risky to have penetrative sex even with your barrier method of choice.

Natural Cycles can also help you if you intend to get pregnant soon. Many people have to come off of hormonal birth control they’ve been on for much of their lives in order to become fertile again, and tracking ovulation can help you figure out what your new, non-regulated cycle looks like. Plus, once you’re actively trying to get pregnant the app can reverse tasks and tell you when you’re ovulating so you can plan to have sex at optimal baby-making times of the month.

But it’s inadvisable if you have certain medication conditions, or if getting pregnant would be problematic for you right now

Even one of the main gynecological experts working with Natural Cycles told The Guardian that the app probably wasn’t right for anyone who was absolutely trying to avoid pregnancy, or for anyone who has irregular lifestyle habits or periods. One of the founders said that the ideal user was someone in a stable relationship who intends to have kids at some point and wants to get off hormonal birth control in the meantime.

If that doesn’t sound like you, you may want to look elsewhere for your contraception.

Some medical conditions can also make it harder for the app to work well. Polycystic Ovary Syndrome, for instance, involves out-of-balance hormones and irregular cycles. The Natural Cycles website notes that people with PCOS will mostly get red days because ovulation is so unpredictable, which means you’ll mostly need to use some kind of protection. It also lists hypothyroidism as a possible problem, since it involves highly fluctuating body temperatures and thus less accurate predictions.

No birth control is 100 percent effective, so choose what’s right for you

Natural Cycles has come under fire for widely-shared articles about a Swedish hospital where dozens of patients seeking abortions had been using the app. It’s important to remember that, while Natural Cycles may not be as effective as many other kinds of birth control, there’s no method that protects every single user forever. There are roughly 6.1 million pregnancies every year in the U.S., and 45 percent of those are unintended. That’s in part because of some simple math. If there are 10 million sexually active people who can get pregnant and they all use a birth control that’s 99 percent effective, 100,000 of them will get pregnant in a single year. Over many years, there will be hundreds of thousands and eventually millions of accidental pregnancies, even with a highly effective form of contraception. The New York Times did an excellent graphic showing this principle over a decade of using 15 kinds of birth control. Only IUDs, sterilization, and hormonal implants produced fewer than 10 pregnancies in a decade of typical use across 100 women. The next-best option, the hormonal injection Depo-Provera, still resulted in 46 pregnancies.

Humans evolved to have babies, and every contraceptive is fighting a battle to keep you from getting pregnant. Picking a method is something you should talk to a medical professional about, but it’s also highly personal. It’s possible that an app like Natural Cycles is the perfect tool to add to your contraceptive arsenal. But before you decide what option is right for you, make sure you know exactly what you’re getting.

Note: This article has been updated to include a disclosure about financial investments, which the author was made aware of after the time of writing