> Many of my friends are convinced that life begins the instant an egg is fertilized, and for that reason are against most birth-control pills, which are designed to prevent conception but as a last resort thin the walls of the uterus (I think that's right) so that even if an egg is fertilized, it doesn't take hold in the uterus and is "spontaneously aborted."

Just a note: Your friends are not quite correct. There are two types of birth-control pills:progestin-only and combination (progestin and estrogen.) The combination pill is used far more often than the progestin-only. It works by preventing ovulation, so there's no egg to be fertilized. They also will thicken cervical mucus, which prevents the egg and sperm from joining. Progestin-only pills do not prevent ovulation; they only thicken cervical mucus.

I can't find any statistics at the moment, but I believe the chances are extremely small with either of the two pills and almost nonexistant with the combination pill that an egg could be fertilized and then prevented from implanting.

~Mina