Can birth control still make you have your period even though you think you may be pregnant?

THIS IS POSSILBE, BIRTH CONTROL IS NOT 100%. MY GODSISTER THOUGH SHE WAS PREGNANT, BUT FOR FOUR MONTHS SHE CONTINUED HER BIRTH CONTROL BECAUSE SHE STILL GOT HER PEROID, AND WHEN SHE WENT TO THE DOCTORS SHE FOUND OUT SHE WAS 18 WEEKS, SO YES IT CAN HAPPEN.