The Pill is designed to interfere with several normal functions of fertility in order to make a woman 1) unable to conceive due to temporary sterility or sperm immobility, and/or 2) unable to carry a pregnancy to term (an early abortion).
The combination pill usually interrupts the release pituitary hormones resulting in no egg being released from the ovary, which prevents pregnancy from occurring. The progestin-only pill, however, has a weaker effect. It generally does not suppress ovulation.
[size=2]Progestins lower the efficiency with which the fallopian tubes propel eggs from the ovaries toward the uterus. This can cause the embryo not to reach the uterus in time to implant successfully. After an ovary releases an egg, the woman’s cycle is controlled by the corpus luteum which normally functions long enough to give an embryo time to implant in the uterus and for the placenta to begin to support the pregnancy. However, hormones from the Pill can cause the corpus luteum to function inadequately, allowing the lining of the uterus to be shed before the embryo can successfully implant.
[size=2]There is no way to know if pregnancy was prevented by preventing ovulation or by impeding a newly conceived embryo. You could argue the chances of the latter are small, ovulation occurs in 2% to 10% of cycles of women taking the Pill. If 60 million women worldwide are on the low-dose Pill, there would be 1.2 to 6 million ovulations per cycle. This is known as breakthrough ovulation, and it is even more frequent with the progestin-only Pill.
[size=2]How come the abortifacient factor isn’t well known to the general public? The Pill manufacturers and many in organized medicine are mainly concerned about the Pill’s medical side effects and its effectiveness in preventing pregnancies and are less concerned about how the drug achieves its effectiveness. Pro-abortion organizations and their lawyers readily admit the early abortion potential of the Pill. In February 1992, writing in opposition to a Louisiana law banning abortion, Ruth Colker, a Tulane Law School professor, wrote, “Because nearly all birth control devices, except the diaphragm and condom, operate between the time of conception…and implantation…, the statute would appear to ban most contraceptives.”
(The above is a post I made on another forum a few years ago -I saved it for reference. Hope this helps.)