Hormonal Birth Control

Hormonal birth control hormones are both contraceptive and “abortifacient.” The artificial estrogen and progestin hormones “fool” the pituitary gland into believing that there is already sufficient stimulation of the ovaries, resulting in the suppression of ovulation in most cycles. Additionally, the hormones may cause the mucus to thicken, making it more difficult for sperm to travel up through the cervix, or they slow tubal motility, which affects sperm and egg movement through the Fallopian tube.
The third action is that they thin the endometrium (inner lining of the uterus) by depleting it of an important sugar, called glycogen. This in turn decreases the thickness of the endometrium, which reduces its blood supply. This situation prevents a blastocyst (a new human being) from implanting in the uterus, thus causing an early abortion. We know that these very early abortions do happen when women take hormonal birth control because studies show that breakthrough ovulation occurs at least two to eight percent of the time, and even more often according to some studies, and yet the Pill has a 99+ percent effectiveness rate.
The side effects of hormonal birth control are too numerous to list, but range from menstrual difficulties to bone density loss, cancers, tumors, heart disease, depression, and surgeries to resolve these negative effects.
See Also:
The Pill by American Life League
The Pill: How It Works and Fails by Pharmacists for Life
Oral Contraceptives and Cancer Risk: Questions and Answers by the National Cancer Institute
The Pill: How Does It Work? Is It Safe? by Couple to Couple League