1. Missing the first Pill in the pack. This is the single most important Pill you'll take all month. Miss it, and your body may start ovulating, and the Pill won't be effective.

2. Missing more than one Pill. The Pill is 98 to 99 percent effective if you take it every day. But when researchers put computer chips into women's Pill packs, they found that 54 percent of takers were missing three or more Pills a month. That's a big deal. "If you skip one, you aren't going to get pregnant. If you skip more than one, particularly with an ultralow dose, you might," says Yale's Dr. Mary Jane Minkin.

3. Being sick. If vomiting prevents you from keeping the Pill down for more than two days, you're in danger of getting pregnant. When this happens, says Dr. Michelle Isley, clinical instructor of OB/GYN at Oregon Health & Science University, the trick women don't know — one that's been confirmed by a new study in the journal Contraceptive Technology — is that they can take their Pill vaginally during those sick days (just insert it like a tampon).

If your illness requires antibiotics, know that only two types — Griseofulvin and Rifampin — have been shown to decrease the Pill's effectiveness, says Isley. "More commonly prescribed antibiotics have not been shown to affect contraceptive hormone levels," she says.

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