Yes

If you take the pill regularly and don't miss any then they are better than 99% effective. But if you have taken antibiotics (which can interfere) or had an upset stomach (eg throwing up or diarreah -sorry can't spell that!) which could mean than a pill didn't get digested, then they might not work and you could get pregnant if you didn't use alternative methods. And the period you get when you are on the pill isn't really a proper period, so I think you could still have that even if you were pregnant.

Wendypops is right and dont forget that some medication over the counter and prescription can affect the way the pill works. Talk to your doctor about it. Good luck