After the Pope’s recent trip to the Philippines, as Rewire reported, he had some advice for Catholic women about family planning: He suggested that there are “acceptable solutions” for couples, so they don’t have to “be like rabbits” and have lots of children. Given the Catholic Church’s prohibition on modern contraceptive methods, however, those “many acceptable solutions” likely boil down to natural family planning.

Once called the rhythm or calendar method, natural family planning at its simplest refers to tracking a woman’s cycle in order to determine her most fertile days and avoiding sexual activity or using a backup method during that time. Today, there are many different methods for doing so, all covered under the umbrella term of fertility awareness-based (FAB) techniques. Some are as simple as counting days and estimating the middle of the cycle—when most women are likely to get pregnant—while others require taking one’s body temperature every day and checking cervical secretions. Though there are a number of low- and high-tech tools (from beads to apps) that women can use, doing this right takes patience and commitment. And, of course, it does not protect against sexually transmitted infections (STIs).

Though FAB methods should be a choice for women—especially those who have decided for whatever reason that they are not comfortable with their other options—it seems clear that the Pope doesn’t understand how difficult and limiting these methods can be.

Counting Days and Calculating Risk

There are only six days during any menstrual cycle that a woman can become pregnant. Most women release one ovum each month, which is only viable for fertilization for about 12 to 24 hours after it is released. Sperm, meanwhile, can live in the reproductive tract for three to five days. The six-day window, therefore, is comprised of the five days before and the day of ovulation.

The tricky part, of course, is determining when ovulation is about to happen—because those are the most unsafe days. According to Contraceptive Technology, 78 percent of menstrual cycles are between 26 and 32 days long; for most women, ovulation occurs right around the middle. If “day one” is the day a woman starts menstruating, most can assume that ovulation occurs somewhere between day eight and day 19.

The easiest FAB method, called the Standard Days Method, is based on this model of ovulation. It requires only that a woman keep track of the first day of her most recent period. Then she counts forward and either abstains from intercourse or uses a back-up method from day eight to day 19.

One popular tool for helping women use this method is called Cycle Beads, essentially a necklace made up of beads of different colors. Women mark the first day of their period by sliding a ring onto the first red bead. They then move the ring forward one bead every day. Days eight to 19 are symbolized with white beads, when women should avoid unprotected sex. When they reach the brown beads at day 20, sex is safer again.

While the Standard Days Method is relatively simple—with or without the beads—it requires 11 days without unprotected sex. Moreover, it only works for women with regular cycles that fall within the 28- and 32-day range.

Another, more specific, method of counting days is called the Calendar Rhythm Method. Before starting this method, a woman must track the length of her cycles for six months and then do a few math problems in order to account for fluctuations in ovulation time. First, she subtracts 18 from the number of days in her shortest cycle, giving her the first fertile day of her current cycle, then she subtracts 11 from the number of days in her longest cycle, which gives her the last fertile day. So, as an example, if her shortest cycle lasted 28 days and her longest cycle lasted 35, she would consider day ten to day 21 to be unsafe. This math has to be redone each month to get the most accurate results for the current cycle.

Monitoring the Body for Signs of Ovulation

For truly accurate results, however, a woman—especially one who has irregular cycles—should keep track of her body’s signs of ovulation too. Before ovulation, women secrete clear, stretchy, and slippery cervical mucus. From an evolutionary standpoint, our bodies are designed to promote pregnancy; these mucus properties can propel sperm toward an egg.

Proponents of one FAB method, called the TwoDay Method, tells women to check for secretions a few times every day by examining their underwear, looking at toilet paper they’ve used, and feeling their vulva. If they notice any secretions they should abstain for two days. Of course, a woman may have to abstain for more than one two-day period using this method because she may have secretions during her cycle that are not actually a signal of imminent ovulation.

More complicated and more accurate versions of this method ask women to look for not just the presence of mucus but its characteristics as well. The proponents of the Billings Ovulation Method, for example, call it “as safe as the pill but natural.” (According to Contraceptive Technology, the perfect-use failure rates of this method and the pill are 3 and 0.3 percent, respectively.) They ask women to check their cervical mucus in order to find their peak fertility day each month—defined as the last day of slippery cervical mucus. Once a woman has identified her peak, she should abstain from sex for the next four days.

Of course, the peak can only be identified after it passes and the cervical mucus is no longer slippery, so there are many days during which this method’s experts advise women to abstain. First, they recommend not having intercourse during heavy menstrual bleeding because the blood can mask the cervical mucus. They then tell women to observe their mucus every day, and give them a chart to record its properties. Whenever women notice changes, the Billings proponents say, they must wait to have sex. If they have slippery mucus again the next day, they should keep abstaining until the day it stops, and then wait four more days. Moreover, Billings Method endorsers say women should only have sex every other day, even during their safe times, so that they do not confuse semen that has remained in the vagina with cervical fluid and mess up their monitoring.

The other major change around ovulation is in something called basal body temperature (BBT) or the temperature of the body at rest. It is lower early in the cycle, rises right around ovulation, and stays high for the rest of the cycle. A woman can understand her typical cycle and know when ovulation has passed by taking her BBT every morning before she gets out of bed using a very accurate thermometer. The temperature can be taken orally, vaginally, or rectally but should be done the same way and at the same time every morning. She then plots it on a graph and looks for an increase of 0.4 degrees or more. Once a woman notices that her temperature has gone up and stayed up by this amount for more than three days, she can consider it safe to have sex again.

Again, this method does not predict ovulation so much as it tells a woman after the fact that she has ovulated. For this reason, experts in these methods often tell women to look at cervical mucus and BBT together—the mucus tells them when their fertility window has started, and the BBT tells them when it has ended.

The Latest Technology

There are many programs that have been designed to help women keep track of and understand the changes in their cervical mucus and BBT. Most of them were originally developed to help women get pregnant—for those who want to conceive, hitting that six-day window can seem like a daunting task. Now, however, there are fertility apps designed specifically for contraceptive purposes as well.

The newest app picking up steam in the media is Natural Cycles, which costs about $70 per year, allows a woman to record the start and end of her menstrual bleeding as well as any spotting. She then takes and records her BBT each morning. She can also use ovulation test strips, which check urine for spiking luteinizing hormone around ovulation. They are optional—Natural Cycles doesn’t appear to include them in the price—but if women enter the data into the app, they can more accurately determine safe or unsafe days. Finally the woman, can (but does not have to) record dates of intercourse, perhaps for her own records.

Raoul Scherwitzl, CEO and co-founder of Natural Cycles, recently told the Huffington Post that the app does all the hard work for women:

The problem that usually comes with [fertility charting] is that when women look at the charts, the data is usually fluctuating with data points going up and down. It can be very hard to look by eye and make objective decisions on whether she’s fertile that day or not. …  We developed an algorithm that analyzes the data so a woman doesn’t need to learn about “What does it mean if [temperatures] go up and down?” She just needs to measure, then we tell her when she is safe or at risk.

The app labels days as red for unsafe or green for safe. Scherwitzl believes it can be 99 percent effective in preventing pregnancy but, according to the Huffington Post, the study supporting this is still under review. Again, though, it is important to remember that BBT only provides part of the picture. It—and the apps that track it—can only be used to suggest when ovulation has already occurred. As mentioned earlier, sperm can live up to five days and, in fact, pregnancy is most likely if the sperm are already in the reproductive tract “waiting” for the egg to be released. So after-the-fact confirmation might not be good enough in determining which days are the safest.

Efficacy and Alternatives

The efficacy rates of FAB methods vary widely; like all other methods, they depend on how well couples use them. According to Contraceptive Technology, those who used FAB methods correctly and consistently saw failure rates between 0.4 and 5 percent. Under typical use, however, the failure rates are between 12 and 24 percent depending on the specific FAB method.

These perfect and typical use failure rates are only slightly higher to those found with other contraceptives. For example, condoms have a “perfect use” failure rate of 2 percent. This means that when used the right way every time, they are 98 percent effective. In typical conditions, though, they fail up to 18 percent of the time, because couples use them incorrectly or forget to use them at all during intercourse. Meanwhile, the pill has a 0.3 percent failure rate if used perfectly, but a 9 percent failure rate under typical conditions.

The fact that the numbers don’t differ immensely means FAB methods can work. But the key to contraceptive efficacy is always consistent and correct use. With FAB, this would mean keeping track of cycles, cervical mucus, and/or BBT, and then never having unprotected sex during potentially unsafe days. While it certainly can be done, it takes a true commitment, a good understanding of reproductive health and one’s own body, and some daily effort.

The Pope is not wrong that Catholic women can use natural family planning methods to space their children and prevent being like “rabbits.” Still, it is unfortunate that this is the only option the Catholic Church allows, as many modern methods of contraception are more reliable and require far less effort. The IUD, for example, has a failure rate of under 1 percent; once it’s inserted, a woman does not have to do anything for up to ten years to reap its contraceptive benefits. Moreover, women in the parts of the world that inspired the Pope’s comment (he was talking about a Filipino woman he met) often do not have access to the kinds of apps and algorithms that do all the work. Like everyone else, Catholic women around the globe should have access to all methods of contraception and should use FAB methods only if that’s what they really want.