Flo. Ovia. Fertility Friend. The past few years have seen an explosion of apps to help women track their menstrual cycles and either conceive or avoid pregnancy. There are currently as many as 1,000 such apps on the market, with some 200 million users around the globe. This year a European regulatory agency even approved an app as a form of contraception.

This comes at a time when women in many countries are rejecting traditional hormonal forms of birth control due to fears of side effects or an increasing cultural emphasis on “natural” health. Some experts applaud the new apps as a way technology can help women take control of their fertility. Others worry they’re just dressing up old, error-prone forms of contraception in high tech clothes.

In February of this year, Natural Cycles became the first app approved for use in the European Union as a contraceptive. The app was created by Swedish physicist Elina Berglund, who was part of the team that discovered the Higgs boson particle. Wanting a non-hormonal way of preventing pregnancy, Berglund developed an algorithm that uses body temperature data and information about menstruation to predict a woman’s fertility window. Natural Cycles works in conjunction with a special thermometer sensitive enough to note changes in basal body temperature—the temperature of the body during rest—which increases after ovulation due to the hormone progesterone. On fertile days, the app gives users a red light, telling them to avoid sex or use contraception. If the light is green, the user can in theory have unprotected sex without worrying about an unwanted pregnancy.

"Women around the world are interested in exploring effective non-hormonal, non-invasive forms of contraception—and now they have a new, clinically verified and regulatory approved option to choose from," Berglund told the press.

A trial involving some 4,000 women showed that about 7 percent of Natural Cycle’s users got pregnant through “typical use”—that is, use with some user errors. This is similar to the failure rate for birth control pills and hormonal birth control patches and injections, and much lower than the failure rate of condoms for typical use, which is about 18 percent. Other apps haven't gone through clinical trials or sought approval as contraceptives, so it's difficult to know how Natural Cycles could fare in comparison.

Menstruation-tracking apps have been popular for several years now, and market research suggests they’re on track to become even more ubiquitous. Flo uses machine learning to predict a woman’s next period based on information she inputs. Period Tracker is an interactive calendar that can be exported to email to share with the user’s doctor. Clue tracks period dates and helps predict the next one, while also keeping tabs on information about mood, pain and more. These apps are essentially high-tech versions of the old paper calendars many women used to keep.

Many of these apps offer predictions about a woman’s fertile window—the five- or six-day period directly before, during and after ovulation, the only time of the month a woman can get pregnant. Some explicitly sell themselves as conception aids. Glow, for example, has pages of user success stories featuring blurry photos of doubled-lined pregnancy tests.

But research has suggested that these apps are often inaccurate. A recent study, published last year in the journal Obstetrics & Gynecology, looked at 53 period- and fertility-tracking apps and websites claiming to predict a woman’s fertile window. Of those 53, only four accurately predicted the window. Some gave windows as wide as 12 days, which is about twice as long as women are actually fertile. Others gave windows as short as four days, when in fact it’s possible for sperm to live for five days in a woman’s reproductive tract. Few apps actually predicted the precise date of ovulation. This date is important, as a woman is much more likely to conceive by having sex on the day before or of her ovulation.

While apps like Glow may claim high success rates in helping women conceive, the data is murky. Is a woman conceiving because the app is helpful? Or is the kind of woman who uses an app more likely to conceive, because of high motivation levels leading to more sex and more careful monitoring of fertility signs?

For many, the stakes are higher when it comes to preventing pregnancy. Though the apps themselves claim to assist in conception, no app before Natural Cycles has gotten official approval as a form of birth control. The app currently has more than a quarter million users, who pay $10 a month or $80 a year. Though the app is only approved in Europe, there’s nothing to prevent women anywhere in the world from using it.

Mary Jane Minkin, a clinical professor of obstetrics, gynecology and reproductive sciences at the Yale School of Medicine, says she started noticing fertility apps gaining traction two or three years ago. She’s wary of women—especially very young women—relying on them as their sole form of birth control.

“Basically all you’re talking about is natural family planning methods,” she says.

The main problem is that basal body temperatures are “notoriously unreliable,” Minkin says. You have to take your temperature early in the morning, before you’ve even gotten out of bed to go to the bathroom. Being sick, hungover or sleeping poorly can affect the result. And, while some women have very regular cycles, many are so irregular that data from one cycle does very little to predict the next.

Minkin thinks apps like Natural Cycles would be appropriate for couples who plan to conceive in the near future, so wouldn’t be upset with an ‘oops’ (Natural Cycle’s creators agree). But for women who absolutely don’t want to be pregnant but are wary of hormonal birth control, Minkin suggests a much older, lower-tech option: the copper IUD. A tiny, t-shaped piece of plastic wrapped in copper, it is implanted in the uterus, where it repels sperm and prevents implantation. Until a few years ago, this kind of IUD was only recommended for women who’d already had a baby. But that restriction is gone, Minkin says. One IUD can prevent pregnancy for 10 years.

“You don’t even have to think about it, and it gives you extremely reliable contraception,” she says.

There have been technological advances that do make natural planning family methods potentially more accurate, Minkin says. Ovulation predictor kits (OPKs) are strips of paper that test urine for signs of the hormones of ovulation, giving women a good idea of when they're most fertile. The results could theoretically be combined with tracking algorithms to make more accurate apps.