How does the Pill work?
The Pill has three mechanisms of action which can easily be looked up in the Physician's Desk Reference.
1) Sometimes, the Pill suppresses ovulation. When this happens, an egg is not released and conception cannot occur. (It's important to read on and find out about the high rates of breakthrough ovulation. When ovulation is not suppressed, pregnancy can occur.)
2) The Pill also works to thicken the woman's cervical mucus which can "restrict" sperm from moving up the reproductive tract toward the egg.
3) One way the Pill causes early abortions is that it interfers with the flexing motions and the cilia movement of the fallopian tubes. These changes slow the transportation of newly conceived child from the fallopian tubes to the womb. Unfortunately, many small babies starve to death in the fallopian tubes because chemicals caused changes that prevented them from reaching the womb in time to be nourished.
4) Another way the Pill causes early abortions: If your tiny baby survives the ride down the fallopian tube to your womb, the Pill will almost always cause the endometrium (the lining of your uterus) to reject your child. Chemical reactions often cause the lining of your womb to become thin, shriveled and unable to support implantation of your newly conceived child.
This means that in almost every case, your new child will not be able to attach to the wall of your womb where he or she would normally live, grow and receive nourishment for 9 months. This means your tiny baby will starve to death and his or her remains will be passed along in your next bleeding cycle. (The "Study of Abortion Deaths Commission" estimates that this happens in women in America who use the Pill approximately 1 to 4 million times each year.)
The chemicals that cause these early abortions are called abortifacients which is the medical term for any chemical agent that causes an abortion.
Taken from this site
http://www.prolife.com/BIRTHCNT.html