One day, in the middle of a college class, Kady Ellison realized that she hadn't taken her birth control pill that morning. Or the three mornings before that. Assuming that the important thing was to just take the pills and wanting to correct her mistake as soon as possible, she downed all four tablets at once as soon as she got home.

It seemed like a reasonable plan at the time. But later, after talking to a doctor, Ellison found out that she'd actually made two mistakes.

“What I should have done was start a new pack and wait a week before indulging in sexual behavior,” the 20-year-old now says. “For about a week after that I was an absolute emotional train-wreck, and it was probably the scariest thing to happen to me in the beginning of my first semester of my freshmen year away at college.”

Chances are, whether you've realized it or not, you've been in Ellison's shoes.

“Most women miss a pill at least once a year and for some it's even more often than that,” says Dr. Kate O'Connell, an OB-GYN with New York-Presbyterian Hospital and Columbia University Medical Center.

Dr. Vanessa Cullins, Vice President for medical affairs at the Planned Parenthood Federation of America, adds that studies have shown only 28 percent of women actually use their oral contraceptives correctly—taking them at the same time, every single day.

Mistakes can make babies

That's a big problem. Hormonal contraception works by tricking your body into thinking that it's already pregnant, thanks to increased levels of the hormones estrogen and progesterone. This keeps you from ovulating, or releasing fertilizable eggs from your ovaries. In order to limit the amount of side effects, there's only so much hormone in each pill—just enough to get you through a little more than 24 hours. After that, hormone levels start to drop back toward normal and you run the risk of ovulating. Which means you run the risk of getting pregnant.

When used perfectly, contraceptives have a failure rate ranging from less than 1 percent to 3 percent, according to various studies. Imperfect use ups those numbers considerably. Dr. Ruth Merkatz, director of clinical development at the Population Council, says that, with “typical use,” the pill has a failure rate closer to 8 percent.

Granted, that's still a far, far lower risk of pregnancy than you'd get without any birth control. On average, women not using any protection have an 85 percent chance of turning up pregnant within a year. But if a woman doesn't want to be pregnant, that relatively measly 8 percent can be the difference between the lifestyle or level of health that she wants and the decisions or changes that she gets stuck with.

That's something Lorri Robinson learned the hard way. When she was 21, she and her husband ended up confined to bed with the flu. She was on the pill, but not long after that week of bed “rest” she found out she was pregnant. Her son is now 28. "I never did get to find out exactly why the BC didn't work. And really, it was moot--it didn't work, now I was pregnant, and that was the issue to deal with."

Follow the rules

It's because of situations like Robinson's that birth control experts say it's important for women to be honest with themselves. “Each woman must do her own self-assessment about what birth control method works for her and with her at that particular time period in her life,” Cullins says. “The pill might work very well when you're young, but as your life gets busier you might do better with an IUD or implant.”

Medications are also a factor—some can interfere with how hormonal birth control works. Of particular concern are certain classes of antibiotics, St. John's Wort, and some of the medications taken to treat seizure disorders. Women should know how anything they're putting in their bodies will affect their birth control.

Finally, it's important to follow the instructions for birth control use and know the proper way to handle an error before it happens. It's also crucial to realize that proper use may vary depending on the method of birth control, or even the type of pill, so you can't assume that you know how to use everything simply because you've been on birth control before. The best thing to do is to talk with the gynecologist or nurse practitioner who prescribes your birth control. He or she can help you wade through the legalese of the package instructions to get to the practical, simple things you need to know.

Tech-Savvy Tricks

Having trouble following your birth control instructions? Technology can lend a hand. Dr. Kate O'Connell, an OB-GYN with New York-Presbyterian Hospital and Columbia University Medical Center, recommends:

Cell phones: Set up automatic text messages or calls through a Web site or your phone's settings. It's a great way to remember to take your pill, and to take it on time.

E-mail: Send yourself a message containing instructions for what to do if you miss a pill or make another birth control mistake. That way, no matter where you are, you don't have to guess at the best way to handle your situation.