You are probably wondering, why is an oral contraceptive an abortifacient?
There are two types of artificial contraceptives: one that prevents the sperm and egg from meeting (like the condom) and another that makes the uterine lining hostile to a conceived life (IUDs and birth control pills). The birth control pill primarily acts to suppress ovulation(release of eggs) by tinkering with the body's negative feedback mechanism for the female hormones that cause the release of eggs by the ovaries. However, this action is not written in stone. It is not foolproof and 100% effective. Eggs can still be released and subsequently fertilized by a sperm, conceiving a new life, a human being.
This is from the product monograph of an oral contraceptive in the US:
"The pregnancy rate in women using conventional-dosage oral contraceptives (containing 35 mcg or more of ethinyl estradiol or 50 mcg or more of mestranol) is generally reported as less than one pregnancy per 100 woman-years of use. Slightly higher rates (somewhat more than one pregnancy per 100 woman-years) reportedly occur with some oral preparations containing 35 mcg or less of ethinyl estradiol, and rates of about 3 pregnancies per 100 woman-years reportedly occur with oral contraceptives containing progestins only." This is from Medscape.com.
So pregnancy still occurs. This only means that an egg can still be released AND fertilized even if a woman uses birth control pills.
Furthermore, the product monograph states:
"Conventional-cycle combination products are usually associated with a shortened period of endometrial proliferation followed by an early but brief and limited secretory activity in the epithelium of endometrial glands. After several oral contraceptive cycles, thinning or regression of the endometrium may occur, resulting in reduced menstrual flow or possible amenorrhea."This can also be seen on Medscape.com
An embryo needs to implant his or herself onto his or her mom's uterine wall, on the endometrium. However, if the birth control pill makes the endometrium hostile by thinning it, where will the embryo implant?
There is a 2007 study in the Obstetrics and Gynecology journal, Obstetrics & Gynecology 2007;110:1003-1009 © 2007 by The American College of Obstetricians and Gynecologists. Its title is "Pregnancy Risk Among Oral Contraceptive Pill, Injectable Contraceptive, and Condom Users in Uganda, Zimbabwe, and Thailand"
(I disagree with the title "pregnancy risk". Pregnancy is NOT a disease.)
However, the abstract states these results:
"The overall 12-month cumulative probability of pregnancy of injectable contraceptive users was 0.6% (95% confidence interval [CI] 0.3–1.0), with similar risks in Uganda (0.3%, 95% CI 0–0.7), Thailand (0.6%, 95% CI 0–1.2), and Zimbabwe (1.0%, 95% CI 0.3–1.7). The 12-month cumulative probability of pregnancy for OCP users was 9.5% (95% CI 8.1–11.0%), with similar risks of pregnancy in Uganda and Zimbabwe (14.6%, 95% CI 11.7–17.4; and 10.2%, 95% CI 8.0–12.5, respectively) but substantially lower risk in Thailand (0.5%, 95% CI 0–1.2). The overall 12-month cumulative probability of pregnancy for women intending to use a given method at baseline was 2.0% (95% CI 1.4–2.6%) for injectable contraceptives, 15.7% (95% CI 14.1–17.3%) for OCPs, and 25.8% (95% CI 23.2–28.4) for condoms. Women in Thailand experienced lower pregnancy risk with condoms (18.4%, 95% CI 11.1–25.7) than in Uganda (29.5%, 95% CI 25.7–33.4), and Zimbabwe (23.3%, 95% CI 19.4–27.2)."
This just goes to show that oral contraceptives are not 100% reliable in suppressing ovulation. Pregnancy can still occur, which means that eggs are still released. These eggs can still be fertilized to conceive a new human being. However, that new human being can die because of a thinned endometrium. This explains how an oral contraceptive pill can cause death of an embryo and thus have an abortifacient effect.