There’s no question that advances in contraception have had a profound impact on our society. Some applaud it. Some resent it. Either way, there’s no uninventing it. Contraception, be it male or female, is a big part of our society and further advances promise to have an even bigger impact.

I’ve talked about the future of contraception before, as well as the potential impacts of those advances. There are a lot of things in development, as I write this, that men and women alike should get excited about. Between better IUDs and more birth control options for men, we’re quickly entering an age where we have unprecedented control over our fertility.

It’s for that very reason that it’s necessary to take a step back and remind ourselves of the existing flaws we have with the current state of contraception. There’s a reason why those advances I mentioned are in development to begin with. What we have now is pretty good, relative to older, more archaic forms of birth control. However, there are a few shortcomings that tend to lead to dramatic, albeit predictable failures.

One of those shortcomings/failures made the news recently and left several dozen women very disappointed, to say the least. According to a story from TheVerge, a recently-developed contraceptive app called Natural Cycles is being blamed for 37 unwanted pregnancies. Anyone who knows anything about pregnancy, female biology, and nature in general probably isn’t surprised.

That’s because the app in question basically made something like this inevitable. According to the article, the Natural Cycles app is supposed to help women who rely on more natural family planning methods. In terms of the actual process, these are the basics:

The app uses an algorithm and measures factors like temperature to determine the period when a woman may be fertile. It’s a popular alternative to hormonal contraceptives like the pill because it lacks side effects.

It’s pretty basic, but the principle makes sense on paper and isn’t based on radically new ideas. The concept of a woman tracking her fertility to determine the times of month when she’s most likely to conceive a child is fairly well-known. It’s most commonly called the “Rhythm Method” and according to the Mayo Clinic, it takes a lot of preparation.

To use this method, a woman needs actively track her menstrual cycle, accurately determine the parts of that cycle where she’s most fertile, and plan her sexual activity around those time-frames. It takes a great deal of physical awareness, as well as a certain measure of discipline, which some people are better at than others.

The Natural Cycles app is supposed to supplement those principles by aiding in that tracking process. It even tries to provide the woman with more data so that she has a better understanding of her body while tracking her cycle. In theory, having perfect knowledge of her biology will ensure a woman knows when she is or isn’t fertile.

It’s that exact theory, however, that makes this news about a surge in unplanned pregnancies so unsurprising. There’s a good reason why this sort of approach to contraception is rarely practiced by anyone other than committed couples and is often discouraged as a general form of contraception.

As with so many other theories, the actual practice of the method doesn’t always work in the real world. That’s because the real world is a lot more chaotic, varied, and unpredictable than concepts on a piece of paper. That applies even more to human biology, male and female.

Now, the appeal of natural family planning like the one Natural Cycles promotes is undeniable. You don’t need to take a pill. You don’t need to have something inserted into your body. Plus, it gives you a chance to really know and understand the workings of your body. For both sexual health and general health, that’s has many benefits.

Unfortunately, the human body is not known for being that transparent. It does not come equipped with a USB port or a wireless interface that provides us with accurate, real-time data about our insides, although that’s one advancement science is working on. Absent that data, natural family planning, even with the aid of Natural Cycles, is going to be either incomplete or flawed.

I don’t doubt that the use of Natural Cycles helps in the process, but there’s a good reason why the same Mayo Clinic that so thoroughly lays out the process of natural family planning also identifies it as one of the least reliable forms of contraception. This is what they had to say about the risks associated with this method.

Using the rhythm method as a form of birth control doesn’t pose any direct risks. However, it’s considered one of the least effective forms of birth control. How well the rhythm method works varies between couples. In general, as many as 24 out of 100 women who use natural family planning for birth control become pregnant the first year. Also, the rhythm method doesn’t protect you from sexually transmitted infections.

Given this information, the fact that some of those using the Natural Cycles app ended up pregnant should surprise no one. The shortcomings of natural family planning are well known and, for the moment, no app is capable of circumventing those shortcomings.

It’s not just that even the most basic activities of the human body are so difficult to quantify and track. Doing so and trying to plan around it assumes all those involved behave with perfect discipline. While I still believe that most human beings are inherently good at heart, I don’t doubt that human beings can also be erratic, irrational, and just plain stupid at times.

There will be moments when a woman misjudges or mistimes her fertility over the course of her cycle. There will also be moments when the passions between a woman and her lover will override whatever discipline they had promised to exercise during that time of the month. Even when people aren’t stupid, they can be overwhelmed by a moment.

That’s the ultimate flaw of natural planning and apps like Natural Cycles. It’s not just prone to human error. It relies on the absence of human error in order to work perfectly. That’s not just flaw. Those are unreasonable expectations that no population of healthy human beings can hope to meet.

It’s sad that several dozen women had to learn this the hard way, even if the outcome was fairly predictable. If nothing else, it should remind us just how much work we need to do on improving contraception, women’s health, and our overall understanding of our biology.