Most birth control pills (such as Alesse, Try-cycling etc. )state a 99% effectiveness of stopping pregnancy when used properly. Which means that one in one hundered women may become pregnant even though they are using birth control.
With an IUD or Norplant, the odds are about 1 in 1000. With the birth control pill, depending on whether or not it was taken correctly and without interfering medication, the odds are about 2-3% per year. With condoms and cervical caps, the effectiveness is 90-98% per year, depending on how well it is used.
If taken properly, the chances are low but actually vary with the method of birth control. For example, the chance of becoming pregnant after a tubal ligation is much less than with using an oral contraceptive. In order of least effective to most effective: condoms (male or female); the pill; a ring or patch; a shot; and sterilization.