Which of those ethical guidelines relates to third trimester abortions, and provides guidance on when they are ethically appropriate?
All of them. They apply to anything a physician does in their practice of medicine.
Let's dig into this.
Principles
I. A physician shall be dedicated to providing competent medical care, with compassion and respect for human dignity and rights.
Assume that a physician holds the same view as Elixir - that a 30 (or 36 or 39) week developed fetus inside the womb is not a person, and thus has no rights and no expectation of dignity... then in what way does this principle prevent or disallow a physician to abort a healthy fetus that represents no known danger to the mother?
II. A physician shall uphold the standards of professionalism, be honest in all professional interactions, and strive to report physicians deficient in character or competence, or engaging in fraud or deception, to appropriate entities.
If a physician believes that a 30+ week developed fetus is not a person and has no rights, does this principle apply to that physician's willingness to abort a health 30+ week developed fetus that presents no known risk to the mother? Would aborting said fetus be viewed as deficient in character, or in competence? Would it be considered engaging in fraud or deception? Would it be considered unprofessional?
III. A physician shall respect the law and also recognize a responsibility to seek changes in those requirements which are contrary to the best interests of the patient.
If the law allows for abortions at any point in the pregnancy, with no restrictions of any sort, would aborting a
30+wk fetus with no known risk to the mom* demonstrate disrespect for the law? If the physician holds the same view as Elixir, that a 30+ safe fetus is not a person and hence is not their patient, then is there any expectation that they would seek changes to that law?
*I'm just going to abbreviate this as 30+ safe fetus from here on out, having to be this repetitively specific every time should be unnecessary
IV. A physician shall respect the rights of patients, colleagues, and other health professionals, and shall safeguard patient confidences and privacy within the constraints of the law.
If the physician holds the same view as Elixir, that a 30+ safe fetus is not a person and has no rights, and thus is not their patient, then would they have any reason to respect the rights of that fetus? In what way would aborting a 30+ safe fetus run afoul of this principle?
V. A physician shall continue to study, apply, and advance scientific knowledge, maintain a commitment to medical education, make relevant information available to patients, colleagues, and the public, obtain consultation, and use the talents of other health professionals when indicated.
In what way does this principle apply to a physicians decision to abort a 30+ safe fetus?
VI. A physician shall, in the provision of appropriate patient care, except in emergencies, be free to choose whom to serve, with whom to associate, and the environment in which to provide medical care.
In what way does this principle apply to a physician's decision to abort a 30+ safe fetus?
VII. A physician shall recognize a responsibility to participate in activities contributing to the improvement of the community and the betterment of public health.
In what way does this principle apply to a physician's decision to abort a 30+ safe fetus?
VIII. A physician shall, while caring for a patient, regard responsibility to the patient as paramount.
If the physician holds the same view as Elixir, that a 30+ safe fetus is not a person and has no rights, and thus is not their patient, then what responsibility do they have to the fetus? In what way would aborting a 30+ safe fetus run afoul of this principle?
IX. A physician shall support access to medical care for all people.
In what way does this principle apply to a physician's decision to abort a 30+ safe fetus?
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The ethical principles you provided are entirely silent on whether or not it's appropriate and acceptable to abort a 30+safe fetus. Doing so does not violate any of these principles
unless there is a law that defines restrictions.