(CNN) An app designed to track fertility and prevent pregnancy in women 18 and older is the first to win US Food and Drug Administration approval to be marketed as a contraceptive, the agency announced Friday . The Natural Cycles app was approved as part of the agency's new Digital Health Innovation Action Plan, designed to fast track approval for new low-to-moderate-risk devices.

"This new app can provide an effective method of contraception if it's used carefully and correctly," said Dr. Terri Cornelison, assistant director for the health of women in the FDA's Center for Devices and Radiological Health, in a statement. "But women should know that no form of contraception works perfectly, so an unplanned pregnancy could still result from correct usage of this device."

Also Friday, the FDA announced approva l of a vaginal ring designed by the global nonprofit research organization the Population Council. Called Annovera, the device is "the first vaginal ring contraceptive that can be used for an entire year," the FDA said.

According to the Population Council, Annovera is the first in a new class of contraceptives. The soft reusable ring combines a new progestin (segesterone acetate) with a widely used estrogen (ethinyl estradiol) to develop a single product designed to be left in place for 21 days and removed for seven days.

"It is indicated to prevent pregnancy for up to a year and does not require refrigeration, which is particularly important for distribution and use in low-resource settings," the Population Council said on its website . In partnership with the pharmaceutical company TherapeuticsMD, the council plans to offer the ring at significantly reduced prices to lower-income women at federally designated Title X family planning clinics in the United States.

First contraceptive app

According to the US Centers for Disease Control and Prevention, between 1% and 3% of women use fertility awareness -- in which a woman's body signs such as temperature are measured to predict ovulation -- as a form of contraception. However, a study from the University of Iowa found that if more women knew about it, one in five would consider it as an option.

Natural Cycles hopes to tap into that market. The app uses sperm survival rates, body temperature and menstrual cycles to predict a woman's fertile days. To use the app, a woman must take her temperature with a basal body thermometer, which provides accurate data to the 10th of a degree, every morning. A red light then warns if there is a risk of pregnancy and to use contraception; a green light says it's safe to have unprotected sex.

Women who are using birth control or hormonal treatments that inhibit ovulation must stop before using the app, the FDA warned because it could invalidate the app's assessment. Women with a medical condition with which pregnancy would be associated with a significant risk , such as high blood pressure, diabetes or kidney disease, should not use the app at all. Nor does it provide protection against sexually transmitted infections.

Natural Cycles was developed by nuclear physicist Elina Berglund Scherwitzl , who was part of the Nobel Prize-winning team that discovered the Higgs boson particle, part of a model that explains the fundamental building blocks of the universe. She and her husband developed and marketed the Natural Cycles algorithm after not finding a satisfactory hormone-free contraceptive option on the market.

Launched in Sweden in 2014 as a fertility app to help women who are trying to become pregnant, the app obtained approval last year as a certified contraceptive from the UK's Medicines and Healthcare products Regulatory Agency.

"This puts the app in the same category as the condom," the company says on its website.

Fertility awareness

According to the American College of Obstetricians and Gynecologists, the app is a form of contraception called fertility awareness . In the first year of typical use of fertility awareness, between 12 and 24 women of every 100 could become pregnant, the group said. If it is used perfectly -- consistently and correctly -- that risk falls to one in five pregnancies per 100 women. The CDC lists fertility-awareness methods as among the least effective of all contraceptive options.

Natural Cycles says the app's reliability for both typical and perfect use has been demonstrated in three studies published in contraceptive journals. Based on those, the company says, the app has a typical-use effectiveness of 93% and a perfect-use effectiveness of 99%.

However, a study published this week in the journal Obstetrics & Gynecology, well after the FDA did its analysis, found "few" studies on fertility awareness methods and said that those published are of "low to moderate quality."

Study co-author Dr. Chelsea Polis, an associate in the Department of Epidemiology at the Johns Hopkins Bloomberg School of Public Health, said the Natural Cycles studies were of moderate quality. In fact, one of the studies incorrectly calculated one measurement, "so we discarded that perfect use estimate in our review." The researchers later corrected the data.

. She points to Polis is concerned about how some devices are promotedShe points to shoddy statistics presented by another manufacturer in the market, which she says has no FDA approval as a contraceptive device but makes misleading marketing claims of being as effective as an IUD.

"The approval of Natural Cycles by the FDA establishes a regulatory pathway for these kinds of devices/apps," said Polis, which she hopes will lead to a "stronger regulatory environment," not a weaker one.

In January, a hospital in Sweden called the app's reliability into question. Södersjukhuset hospital in Stockholm said that at least 37 women sought abortions at its facilities after using Natural Cycles as a birth control method. Natural Cycles confirmed the report, saying at the time that the numbers "are not surprising given the popularity of the app and [are] in line with our efficacy rates," adding that unwanted pregnancies are an "unfortunate risk with any contraception."

Both the app @NaturalCycles and the EU should know better. You DO NOT launch an app that tracks female fertility based *solely* on basal temperature and you certainly DO NOT approve it as a method of contraception. This is so utterly irresponsible. — Anna Rascouët-Paz (@rascouet) July 25, 2018

Some women have gone public with their distrust. One wrote about her experience with Natural Cycles in an article in The Guardian headlined "I felt colossally naive': the backlash against the birth control app." After four months using the app, she became pregnant and had an abortion.

"As someone who didn't report my own pregnancy last year, keeping it secret even from my parents, I wonder how many more there have been," Olivia Sudjic wrote. As it turned out, Sudjic had polycystic ovary syndrome, a hormonal imbalance that causes irregular menstrual cycles, infertility and pelvic pain.

Global family planning

The Annovera vaginal ring, which will be available as early as the fourth quarter of 2019, is the sixth contraceptive device developed by researchers at the Population Council. The council estimates that more than 170 million around the world are using one of its other devices, which include a copper IUD, contraceptive implants and a contraceptive vaginal ring for breastfeeding women.

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Vaginal rings are not barriers like diaphragms. They work like birth control pills, releasing hormones into the body that either stop sperm or inhibit ovulation; most last about four months. Annovera uses a new hormone mixture and is effective without refrigeration for up to a year.

When used properly each cycle, by leaving the ring inside the vaginaI canal for 21 days and removing it for seven days, Annovera was shown to be 97.3% effective in clinical trials. The device was tested in women between the ages of 18 and 40 who used it for over 13 menstrual cycles. According to the council, a subset of women in the trials found the device easy to use, convenient and comfortable, even during sex.

The device comes with a boxed warning about increased cardiovascular risk when used while smoking, the council said, and has not been evaluated in women with a body mass index greater than 29.

According to the CDC, more than 43 million women in the United States are at risk of unintended pregnancy. The birth control pill is the most common form of pharmacological birth control; studies show that four in five American women use them. Yet nearly 30% of all users stop because of side effects that include nausea, weight gain, sore or swollen breasts, spotting and mood changes, according to research from the CDC.

CNN's Nadia Kounang and Sara Ashley O'Brien contributed to this report.