For the first time, an app that relies on fertility awareness — also sometimes known as the temperature rhythm method — will be able to market itself in the US as a way to prevent pregnancy.

The Food and Drug Administration announced that it's given the green light to Natural Cycles, an app that you can use to track your natural menstrual cycle and body temperature to predict when you're fertile and when you're not.

To use the app, you need to take your temperature every morning with a basal body thermometer, which is a specialized thermometer that can detect very small body temperature changes. Just after ovulation, the basal body temperature rises by 0.4 to 1.0 degree.

The data are entered into the app, which uses that information to predict ovulation. The user can either abstain from sex or use barrier protection, such as a condom, to prevent pregnancies.

For decades, women have used natural body changes, including temperature and cervical mucus production, to predict ovulation. By doing so, you can time sexual activity and birth control in ways that either increase or decrease the chances of getting pregnant.

Natural Cycles claims to be 93% effective with typical use, according to research tracking 22,785 women through 224,563 menstrual cycles across two years. With perfect use, the app company says the method is 99% effective.

According to the Center for Disease Control and Prevention, traditional "fertility awareness" is one of the least effective methods of birth control, with 24 in every 100 women using this method becoming pregnant within a year of typical use. By contrast, the IUD is more than 99% effective and the pill is 91% effective.

The company said the app "may not be the most suitable form of contraception" for people with irregular menstrual cycles, such as those with polycystic ovary syndrome (PCOS). About one in 10 women of reproductive age have PCOS, which can cause irregular periods, infertility, and other issues.

Dr. Gillian Dean, senior director of medical services at Planned Parenthood Federation of America, told BuzzFeed News there are three main benefits to fertility awareness: It's either low or no cost, it has no side effects, and it helps people learn about their bodies.

There are several methods of fertility awareness, including temperature checking, which Natural Cycles uses, and checking vaginal discharge. The cervix produces a clear fluid with a consistency of egg whites around the time of ovulation. Using more than one ovulation-detecting method is the safest bet when it comes to predicting fertility.

Dean also said it's important to chart temperature for at least three months before relying on that method for birth control.

"While there is a risk of unintended pregnancy with any method, the higher risk with [fertility awareness methods] means this may not be the best method for someone for whom an unintended pregnancy would be unacceptable," said Dean.

"To use FAMs effectively you need to be very knowledgeable about your menstrual cycle, when you’re ovulating, and when you’re fertile, so that you understand when it’s safer to have sex without risking pregnancy."

Natural Cycles markets itself as a digital yet natural form of birth control and told BuzzFeed News it has more than 900,000 users around the world.

"We are delighted to be the first mobile app cleared by the US FDA as a form of birth control," a Natural Cycles spokesperson told BuzzFeed News.

"Our mission at Natural Cycles is to pioneer women’s health with research and passion, and this milestone marks a very important step in that journey."