Just wanted to add in on this thread:
Birth control pills: IS NOT a mortal sin if one abstains from the sexual act while using it (for purely medical reasons) and up until the time the medication is completely out of one’s system. If you are taking BCPs and not abstaining, then you are committing a mortal sin.
Other types of birth control (Non-NFP): Are mortal sins. This includes vasectomies, getting your tubes tied, and condoms. Anything used to prevent the miracle of new life in a marriage is a mortal sin. NFP is NOT a mortal sin because, while it is used to postpone the creation of new life, it is not used to prevent new life. NFP, when used properly, is always open to new life.
Hope this clears up any misunderstandings one may have on birth control.