Do the birth-control pills prevent fertilization or they just prevent the fertilized egg to attach to the uterus?

Multi - ways: Birth control pills work in many ways depending on the type (ie Progesterone only or combination pills). The main function is to stop ovulation from occurring. If no egg is released then there is no egg to fertilize. If this fails and an egg is made and released - the secondary defense is thinning the lining to prevent embryos from implanting. They also effect cervical mucus.