About half of all American teens have had sex by the time they turn 17, according to a 2013 study, and many are woefully underprepared.

A quarter of young women responding to a Guttmacher Institute survey in 2009 had "low" knowledge about contraception. And CDC data for 2006 to 2010 found that 83 percent of sexually experienced teen girls reported no formal sex education until after they'd had sex for the first time.

These days, many young people are turning to a slew of mobile apps that aim to provide statistics, education and support. Some of these apps are teaching teens about sex and recommending birth control; others are helping young women better understand their menstrual cycle.

Alyssa Padron, 20, starting using iPhone apps to track her period about two years ago. She dabbled with the Period Tracker Lite app, before switching to the Eve app after it launched in July. The Eve app resonated with her as it is designed for women her age who are not trying to get pregnant.

“I got my first period pretty late, when I was 17, and started tracking right away,” says Padron. “You obviously can’t control what your body does, but I like to feel prepared and to know what to expect.”

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Dr. Jeanette Lager, an OB/GYN at UCSF Medical Center, says she sees a lot of young patients who track their menstrual cycles using their phones. She says it "creates a more interactive conversation" with patients.

“It makes it more efficient for doctors to take a quick glance," Lager says, "and see what a patient’s cycle has been like for the past six months."

Period tracking is harmless enough, but Lager and other doctors are wary of apps that advise women on birth control options. Each patient's medical history is different, and selecting the optimal birth control method often requires an in-person discussion with a professional.

Logging Data and Asking Questions

Eve aims to teach young women like Padron about their sexual health and fertility. Beyond period tracking, it allows women to log their sexual activity, moods and symptoms. More importantly, it provides in-app communities where women can discuss sex-related topics and talk about the pros and cons of various birth control methods.

One in-app community is the "sexplanations" section, where users share their thoughts and experiences on range of topics, and are encouraged to “ask anything.”

“It’s a great place to go for sex advice," said Padron. "Questions range from ‘what is this discharge?’ to ‘which dress do you like better?’”

A new app called Eve helps women avoid getting pregnant. (Glow )

Formerly known as Ruby, Eve is the latest product from a company called Glow, which was started by PayPal cofounder Max Levchin in an effort to “demystify womanhood” with data.

Glow's latest version of Eve allows women to rate sex, and log it the morning after as opposed to the night of. They can also log exercise, indulgences like ice cream and, in an effort to make the app less heteronormative, sex with non-male partners.

Padron says app makers should make an effort to appeal to young audiences with an accessible tone that doesn't feel too scientific. She prefers using Eve as it asks explicitly about sex and includes "cute little pictures that look like emojis."

The birth control section of the app was created in partnership with Bedsider, an online birth control support network, and includes information about an array of birth control options.

Overall, Lager is optimistic about the role mobile devices can play in educating young women about birth control and sexual health.

“I think the main risk or danger of an app like this," Lager says, "is if someone were to go straight from the app to moving forward [with birth control], without having a discussion with a professional."

In some cases, she continued, mobile apps may prove to be a vital resource for young patients who are nervous about discussing their sexual health, or have difficulty accessing a provider.

“Any opportunity for education is vital," Lager says. "And teenage girls are on their phones all the time."

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'Demystifying Womanhood with Data'