One pill is not likely make a difference. Remember that, birth control pills are not 100% effective, even if taken exactly as prescribed. So pregnancy is always a small possibility.

More important is when you started taking the pills, since that determines whether, and how long you need to use a backup birth control method.

This is the normal instruction:

• "You can start taking them on the first day of your period, in which case you won't need backup birth control.
• You can start taking them the Sunday after your period starts, in which case you will need backup birth control for seven days.
• You can start taking birth control pills on the day they are prescribed, in which case you will need to make sure you're not pregnant and you will need to use backup birth control for the first month."

As far as missing one pill and still being okay, here is information about that. Good luck.