Informed Consent?

I've been doing some reading about exactly how hormone based birth control works.

In summary, according to multiple references throughout The Physician's Desk Reference, which articulate the research findings of all the birth control pill manufacturers, there are not one but three mechanisms of birth control pills:
       1. inhibiting ovulation (the primary mechanism),
       2. thickening the cervical mucus, thereby making it more difficult for sperm to travel to the egg, and
       3. thinning and shriveling the lining of the uterus to the point that it is unable or less able to facilitate the implantation of the newly fertilized egg.

The first two mechanisms are contraceptive. The third is abortive.

When a woman taking the Pill discovers she is pregnant (according to The Physician's Desk Reference's efficacy rate tables, this is 3 percent of pill-takers each year), it means that all three of these mechanisms have failed. The third mechanism sometimes fails in its role as backup, just as the first and second mechanisms sometimes fail. Each and every time the third mechanism succeeds, however, it causes an abortion.
Does the Birth Control Pill Cause Abortions? by Randy Alcor (condensation)

I'm not trying to debate if abortion should be legal or not. I'm simply saying that there are those that would choose to have an abortion and there are those that would not. For those that would not, this is extremely troubling.

The other trouble is that many medical care providers do not use the same definition of "pregnancy" that is commonly understood by most people (and in most widely available dictionaries).

From a medical point of view, however, pregnancy does not occur at the moment of conception. It occurs, instead, when an embryo (a fertilized egg that has divided over the course of a few days) attaches itself to the woman's uterus, a stage known as implantation. It is at implantation that a woman's hormonal system begins to respond to her embryo, a response that initiates a cascade of dramatic physiological changes in her body. This means that if a sperm fertilizes an egg after a couple has intercourse, but the fertilized egg never implants inside the woman's uterus, then the woman - from a medical point of view - was never pregnant. Therefore, she can be described as having menstruated, rather than as having experienced a miscarriage or a spontaneous abortion.

Some forms of what we call birth-control implicate the distinction between the pro-life definition of pregnancy and the medical definition of the same. For example, the I.U.D. (or intra-uterine device) can operate by preventing a fertilized egg from implanting in the wearer's uterus (though it can also work by preventing conception in the first place). When it prevents implantation, an I.U.D. has - necessarily - not prevented conception (and, if I were a pro-life advocate, I might accordingly say that in such instances, it does not literally fit the definition of "contra-ception").
When Does Pregnancy Begin?: A Federal Appeals Court Decision Implicates a New Abortion Question by Sherry F. Colb

The view that pregnancy begins at implantation is the view held by the American College of Obstetricians and Gynecologists (ACOG). When your medical care adviser tells you that various birth control methods "do not disrupt an existing pregnancy" (as stated in the World Health Organization's "Family Planning: A Global Handbook for Providers") realize that you both may be using the same word, "pregnancy," but the definition is not the same.

How can patients be expected to give informed consent if a word with a the commonly understood definition is being used with a definition crucially different?

Neither ACOG definition has been consistently adopted by its members whose definitions are more consistent with lay and embryologist definitions. Potentially, the process of informed consent is jeopardized by these ambiguities. The ACOG is urged to reconsider its definitions.
Informed consent and the redefining of conception: a decision ill-conceived?  by J.A. Spinnato (abstract)

People most likely ask the question "Will this method of birth control harm a pregnancy?" are most likely people who would consider pregnancy to begin at fertilization and would consider any post-fertilization effects, such as inhibiting implantation, to be harmful to a pregnancy. To dismiss the commonly understood definition of "pregnancy" and play a game of semantics does not allow for informed consent and is poor care indeed.