Millions of women are turning to their smartphone to track their menstrual cycles.
There are almost 100 apps to help them. Some are pretty simple, designed primarily to help women track their periods and predict ovulation. But others have more detailed features and bill themselves as an effective means of natural contraception.
One app, Natural Cycles, used its own basal body thermometer and fertility prediction algorithm to become a certified method of contraception in Europe last year. Natural Cycles is currently awaiting a decision from the U.S. Food and Drug Administration on whether it can market the app as contraception in the U.S.
In December, Natural Cycles released a study demonstrating that its app is 99 percent effective with perfect use and 93 percent effective with normal use, which is better than the pill. However, the app sparked criticism after 37 Swedish women got pregnant while using Natural Cycles as their primary form of birth control.
Experts like Dr. Ruth Ann Crystal are skeptical. The Bay Area-based obstetrician and gynecologist said she doesn't believe a fertility tracking app could ever be 99 percent effective.
"I would say all OB/GYNs would agree with me on that," she said. "There's just too much variation in the human body as to when people ovulate, be that from stress or travel or being up late or sickness."
Crystal said the Natual Cycles study was problematic from the start because the founders of the company co-authored the paper alongside members of its advisory board. Now there's a backlash from women who trusted the numbers but still wound up pregnant.
Francine Aguerre Herrera, a San Francisco-based acupuncturist, said she started using an app because she wanted to regulate her cycle in a more natural way.
"I went ahead and went off birth control with acupuncture and herbs, and started doing it the old-fashioned way, where you track your cycle on paper," she said.
Five years ago, Herrera discovered Kindara, which she says helps her track her cycle more efficiently and responsibly. She calls herself "living proof" that you can use the app as contraception, though she admits that doctors are usually shocked to hear she's using it as her primary form of birth control.
Apps like Kindara promote fertility awareness, a natural contraceptive method that, at its most rigorous, requires users to track their cycle length, basal body temperature and the appearance of cervical fluids. For women seeking to prevent pregnancy, Kindara doesn't make fertility predictions but instead requires users to interpret their own fertility charts according to a specific set of guidelines outlined on its website.
Clinical trials of the fertility awareness method demonstrate low failure rates for perfect use. However, there's a lot of diligence involved and education needed to correctly interpret the body's signs and rhythms, Crystal said.
"I would worry that people don't remember to do all these steps for these apps," she said. "It's going to be hard to be perfect and do it on clockwork always, and that's one of the reasons these methods won't work as well."
Whether or not daily tracking is feasible, doctors and some fertility awareness advocates find common ground when it comes to relying on an app's predictive algorithms.
"Your phone is kind of averaging everything out, but it's not really tracking you," said Laura Brown, a women's holistic healer and a user of the Kindara app.
Some fertility awareness advocates like Brown encourage women to educate themselves to the point where they can trust their own body and interpretation of the data over an app like Natural Cycle's predictions.
But with so many women already tracking their cycles on their phones, and a desire among many to avoid hormones or implants, Crystal predicts that the prevalence of app-based family planning will continue to rise.
She emphasized the need for more randomized clinical trials. And she said it's unlikely that an app could ever outpace other traditionally reliable methods of birth control.
"Do I think that someday an app such as this could be as effective as the pill or an IUD?" she said. "No, I don't. I really think that the human body has too much give and take."