Long before apps, there was the rhythm method.

With the rhythm method, a woman tracks her menstrual cycles on a calendar to pinpoint when she is ovulating and most likely to conceive — and so avoids sex on those days. This type of natural family planning is far from foolproof, with as many as 24 in 100 women who practice it as birth control getting pregnant in the first year.

Over the years a number of apps (Kindara, Period Tracker and Ovia, among dozens of others) have popped up to help women better understand their fertility cycles. These apps simplify data-logging, help women “learn” their cycles and typically factor in other information like body temperature or the consistency of cervical fluid, both of which can indicate ovulation or the time you're most likely to become pregnant.

Most of the apps are promoted as pregnancy planning tools and that's how many women use them. OB-GYNs such as Dr. Jamil Abdur-Rahman, clinical instructor at the Rosalind Franklin University Chicago Medical School, often recommend the apps for patients trying to get pregnant.

None of these types of apps have been approved by the Food and Drug Administration to be marketed as contraception, until now. Earlier this month, Natural Cycles was cleared by the FDA as a method of contraception. The app, which primarily markets to women seeking a natural form of birth control, contains an algorithm that calculates the days of the month a woman is likely to be fertile based on daily basal body temperature readings and menstrual cycle data.

"Consumers are increasingly using digital health technologies to inform their everyday health decisions, and this new app can provide an effective method of contraception if it’s used carefully and correctly," the FDA wrote in its release.

Certain religious beliefs may preclude patients from using birth control, and for them, Natural Cycles may be a good choice of contraception, just as it may be for women who cannot tolerate hormonal treatment or an intrauterine device.

The FDA approval is based on the efficacy rate of the app when used as a contraceptive measure.

"The clearance was granted based on the evidence provided in prospective clinical studies based on real-world data from 15,570 women, in which Natural Cycles was shown to be 93 percent effective with typical use,” Natural Cycles founder and CTO, Dr. Elina Berglund Scherwitzl, told NBC News BETTER in an email.

OB-GYNs were dismayed over the FDA ruling, saying that women who want to avoid pregnancy should use any fertility app with extreme caution. A 93 percent efficacy is low in comparison with other birth control options like the birth control pill (98 percent effective) and IUDs (more than 99 percent). But that’s not the main concern that OB-GYNs have.

“Consider that this 93 percent number comes from a controlled study where participants were being extremely diligent. What happens in the real world?,” says Dr. G Thomas Ruiz, an OB-GYN at Memorial Care Medical Group.

Dr. Nicole Swiner, a family medicine doctor, stresses that “smartphones aren't perfect, people don't always plug in the right date or data for things, and this is not the best way to prevent pregnancy.”

Natural Cycles requires data and diligence

Natural Cycles, which costs $79.99 a year and includes a basal body thermometer, requires daily input to inform women about when it's safe to have sex, or avoid sexual activity to prevent pregnancy. Natural Cycle's Berglund Scherwitzl says the app contains an "algorithm that can accurately determine a woman’s daily fertility based on changes in her basal body temperature — which increases after ovulation.”

The proprietary algorithm is designed to account for a variety of factors including "sperm survival, variation in cycle length, temperature fluctuations and the length of the follicular and luteal phase," according to the company. Based on that information, the app will display a "red day," alerting you to avoid sex, or a "green day" indicating that it's safe to have to intercourse.

Fertility cycles can be hard to predict

Since Natural Cycle’s algorithm is proprietary, it’s unclear exactly how it learns a woman's fertility cycle, which can be extremely unpredictable. This is why even the most diligent rhythm method users have gotten pregnant, and why this app isn’t a contraception method that many OB-GYNs would jump to recommend, especially for young women and teens.

While some women have cycles that range the average 28 days, many have more erratic cycles. Even women who have a history of predictable cycles could be thrown off by a list of factors, says Swiner.

“Sleep, stress or recently coming off of [hormonal birth control] can create irregular periods,” Dr. Swiner says. Dr. Abdur-Rahman adds that even aggressive exercise can throw off your cycle.

This app isn’t for everyone, but it shouldn’t be dismissed

The OB-GYNs I consulted said that despite their reservations about the FDA-approved contraceptive app, they do not dismiss Natural Cycles, so long as patients understand the risk of unwanted pregnancy.

Kristi L., a 32-year-old woman who works in politics and asks to not disclose her last name, has been using the app Natural Cycles as contraceptive for months after a series of negative experiences with hormonal birth control — with the guidance of her doctor.

“I found [Natural Cycles] out of necessity,” she says. “Birth control pills were making me feel like total garbage — both emotionally and physically. I wanted a metal IUD to avoid hormones, but because I am allergic to nickel, there was a high chance that I would have a bad reaction to the copper IUD as there can be trace amounts of other metals in it."

If you want to use a natural form of contraception, consider this advice

If you’re thinking of switching from from the pill or an IUD to a fertility awareness app like Natural Cycles, consult your doctor first. The OB-GYNs I spoke to shared these important tips.

Be diligent. These apps may cost you, but that doesn’t mean they do the work for you. Log all your data as instructed.

These apps may cost you, but that doesn’t mean they do the work for you. Log all your data as instructed. Take your temperature first thing. You must take your basal body temperature at the same time every morning, as soon as you wake up, with no exceptions. Don't eat or drink anything first. “Even the slightest thing (like going to pee first), can affect your basal body temperature and then all bets are off,” says Dr. Ruiz.

You must take your basal body temperature at the same time every morning, as soon as you wake up, with no exceptions. Don't eat or drink anything first. “Even the slightest thing (like going to pee first), can affect your basal body temperature and then all bets are off,” says Dr. Ruiz. Don’t trust it if you’re sick. “If you have a cold or any infection, or are on antibiotics, your basal body temperature will likely be affected. You have to be perfectly healthy,” adds Ruiz.

“If you have a cold or any infection, or are on antibiotics, your basal body temperature will likely be affected. You have to be perfectly healthy,” adds Ruiz. Don’t use this on birth control. Birth control throws off your basal body temperature so this won’t serve much purpose if you’re taking the pill.

Birth control throws off your basal body temperature so this won’t serve much purpose if you’re taking the pill. Give your body time to adjust. Wait for three menstrual cycles before having unprotected sex.

Wait for three menstrual cycles before having unprotected sex. Don’t use this with multiple partners. This method doesn't offer any kind of physical barrier, so it doesn’t protect against sexually transmitted diseases. If you’re not in a committed relationship, or even if you’re unsure how you would deal with an unplanned pregnancy, use condoms, too.

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