What is a birth control pill (BCP) and how does it work?
Birth control pills are acknowledged to have two other main effects: They thin the inner lining of the uterus (called the endometrium), depleting it of glycogen (a type of sugar) and blood supply, and BCPs may thicken the cervical mucus, making it more difficult for the sperm to travel up through the cervix. Though this latter effect is claimed by BCP manufacturers, the evidence for it is weak4, 5 and not strongly supported by the rabbit model.6 Of course, BCPs could not cause abortions if they always stopped ovulation so this needs to be the first issue that is raised. A clear indication that ovulation will occur in women taking the BCP is provided by noting what the BCP manufacturers state in the PDR (Physician’s Desk Reference, 1998).7 The “efficacy rate” table for each BCP claims a “typical failure rate” of about 3%. The PDR defines “typical failure rate” as the rate of annual pregnancy occurrence in “typical couples who initiate use of a method (not necessarily for the first time)
