Abortion (as humans are concerned) consists in deliberately causing the death of an unborn human.  Usually the death is brought about by direct killing or by causing a miscarriage.
Some facts about abortion in the US:

1. 1.3 million Americans have abortions every year. (source: New York Times Aug. 8,1996)
2. The overall abortion rate in the United Stateshas dropped about 10% in recent years.(source: New York Times Aug. 8, 1996)
3. Based on a 1994-95 study, about half of all U.S. women will have an abortion at some point in their lives.(source: New YorkTimes Aug. 8, 1996)
4. The proportion of teenage women having abortions dropped from 25.5% in 1987 to 21.5% in 1995. (source: New York Times Aug. 8, 1996)
5. 6 in 10 women who have abortions say they were using contraceptives when they became pregnant. (source: New York Times Aug. 8, 1996)
6. 99% of all abortions are performed in the first half of a pregnancy's gestation period.
7. .04% of abortions are performed 26 weeks or more into the gestation period.
8. Catholic women have an abortion rate 29% higher than Protestant women. (source: New York Times Aug. 8, 1996)

Two issues are centrally discussed:
 i) is the fetus a person?
 ii) if it is, how do we solve conflicting claims?

A) The status of the fetus.

1. An argument for the immorality of abortion which immediately comes to mind is  this:
 i. the fetus is an innocent human being
 ii. killing an innocent human being is morally wrong
 iii. hence, abortion is morally wrong

Problem:
Premise (ii) fails to distinguish between ‘being genetically human’ and ‘being a  person’.  What's wrong primarily is killing persons (beings with moral rights and  duties), and killing humans is wrong insofar as they are persons.  That is, belonging  to the species homo sapiens is not, per se, morally significant unless it is  connected with being a person.
 

2.  What the necessary and sufficient conditions for personhood are, is a matter of controversy.  It is clear that any individual who is self-conscious and acts autonomously on the basis of a set of moral beliefs, that is, who is a center of autonomous choice and evaluation, qualifies as a person (ask yourself when you would call a being from outer space a person).  Obviously, any normal adult human being qualifies as a person under these criteria.  However, it is perhaps possible to relax the criteria somewhat.  Feinberg claims that five necessary and sufficient conditions are embedded in the commonsense notion of personhood:

1. being conscious , e.g. aware of one's surroundings.
2. being conscious of itself, i.e. being able to think of oneself as oneself at least at a rudimentary level.
3. being able to reason and know, e.g. plan, understand at least at a rudimentary level.
4. being  a sentient being, e.g. feel pain/pleasure.
5. being able to have emotions.

NOTE: (1)-(3) are less problematic than (4)-(5). Perhaps (1)-(3) are enough for personhood.

• A person in the commonsense use of the notion needsn't have moral duties insofar as needn't even have a rudimentary moral system.  For example, a normal two year baby is a person under these relaxed criteria, but can hardly be thought of as having moral duties.
• A person in the moral sense needn't be one in the legal sense (the latter has legal rights/duties on the basis of positive law and can be non-living, e.g. a corporation).
• Questions:
• at what point are (1)-(5) so relaxed as to be satisfied by some (many?) animals?
• Children and animals

3. Proposed extensional equivalents of moral personhood.

• homo sapiens: all and only humans at any stage of their development are persons.

Problems:
1. it's not clear why our species should be singled out.  It's possible that other beings are persons (God, aliens, other species); nor is it clear why a man who   cannot have (1)-(5) at all should be a person.  The position seems guilty of unwarranted homocentrism, i.e. speciesism.
2. if zygotes are persons, then we should spend billions to avoid natural miscarriages (about 65% of cases).  There would also be about 15% of (often) grossly handicapped babies instead of about 2%.
3. any procedure preventing implantation of zygote to uterine wall would be murder.  Hence, some drastic steps (monthly tests for women?) might be justified.
• modified species: all and only members of any species normally having (1)- (5), whether they have (1)-(5) or not.

Problem: there seems to be no reason to include members not satisfying (1)-(5).
• strict potentiality: all and only those beings which actually have (1)-(5) or have within themselves the capacity to develop into beings which actually have (1)-(5).

NOTES:
• Hence, while spermatozoa and ova don't qualify, the embryo does
• This is the standard position of those who consider abortion murder.
Problems:
• Using the morning-after pill, which flushes the embryo before implantation, is the moral equivalent of, say, killing a sleeping adult.
• It seems that beings which have (1)-(5) potentially  are merely potential persons, and consequently have at  best merely potential rights. But a potential right is not a right any more than a  potential meal is a meal.  A 9-year old citizen has a potential right to vote, but we  don't admit her in the voting booth.

Reply:
having (1)-(5) potentially confers not actual full rights, but actual weak rights  which grow stronger and stronger the closer the subject comes to having (1)- (5).
Duplication: The argument moves from almost qualification to a right to a weak  version of the right, and the move is illicit.  The day before inauguration the would-be president has no right (not even a weak one) to command the armed forces.
NOTE: although a potential right is no right at all, and hence not a weak right, one might argue that the closer the subject is to having an actual right, the more interest we should have in the preservation of the subject in the light of the right it almost has.

• in some cases it may be permissible (perhaps even morally required) to kill severely handicapped babies (i.e. before they become persons).
• care should be taken to avoid, when possible, fetal suffering (apparently, this is very rarely done).

• A normal adult doesn't satisfy (1)-(5) when he's asleep.  But certainly a sleeping adult is a person.

Reply: there are two levels of actuality, first and second.  First actuality consists in actually possessing a capacity (e.g. one knows French) without    exercising it (one is asleep).  Notice that first actuality is different from mere potentiality.
• A neonate does not have (1)-(5).  Hence, he's not a person, and consequently infanticide is justified.

Reply:
Infanticide is wrong not on the basis of right to life, but on basis of other considerations: infaticide ultimately produces a loss of respect for adult human life, which utilitarian (or egoistical!) considerations show must be avoided.   The same sort of considerations show that although abortion is not murder, society may regulate it.
Duplication: There's little evidence that infanticide leads to a cheapening of adult human life and the consequent corrosion of the general prescription against killing adults as long as the infanticide is carried out before the infant is considered a member of the community , e.g. Japanense town of Nakahara in 18th and 19th century, or some Indian villages which practice infanticide on baby girls. (This last case, however, may cut both ways: ask yourself: "What's the status of girls and women in such villages?")
So, it looks as if the problem remains.

B) The problem of conflicting claims
Assume, for argument's sake, that the fetus is a person.  Then, abortion is terribly wrong.  However, a pregnant woman has rights of her own which can come into conflict with those of the fetus, in which case there is a conflict of rights.  There are three basic rights on which the right to have an abortion can be based: property rights on one's own body; right to self-defense; right to bodily autonomy.

1. Property rights over one's body.
There are 3 main arguments centering on the property rights a woman has over her  body:

1. The fetus is like an organ or a growth; since these may be eliminated, so may  the fetus.

Problem: the fetus is not a mere growth, but an independent entity temporarily growing in the mother.
2. The fetus is a part of the woman's body; hence it's her property, like a watch  or a house.

Problems:
• this is incompatible with the assumption that the fetus is a person;
• fetuses not bought and sold, or used as collateral.
3. A woman's body as her property; hence she can expel an unwanted fetus in the same way in which one can expel an unwanted guest from one's house.  Problem: one may not throw out a weak guest who was brought unconscious in the house into the freezing cold to certain death.

2. The right to self defense
The idea here is that the intentional killing of a person is justifiable in cases of  self-defense, e.g. survival, maiming, rape, severe beating, etc. But pregnancy  and/or having an unwanted child can bring severe harm to a woman (think of cases  of rape, much too many children, very difficult pregnancy).
Problem:
the standard cases of self-defense involve killing an aggressor; but the fetus is not an aggressor (not even an innocent one).
Answer:
It's justifiable (though regrettable) to kill the innocent shield of a threat  (Thompson’s case of the tank with the baby).

NOTE: Hence, the right to self-defense allows abortion only if a woman's life is at substantial risk.  However, abortion to have a happy life is not justified because one cannot kill an innocent person in order to be happy.

3. The right to bodily autonomy
In the case of Thompson's plugged-in-violinist, it would be supererogatory to  remain plugged.  Hence, in cases of unwanted pregnancy, it would be supererogatory, though not morally obligatory, to bring the fetus to term (to remain plugged).
Problem:
There's a deep disanalogy between abortion and the violinist case, for the woman, by  voluntarily having sex, is often co-responsible for the existence of the fetus.  But  voluntariness entails responsibility, and therefore the woman's responsibility  toward the fetus will be proportional to the degree of voluntariness.  One can  distinguish several cases:

1. rape (totally involuntary).
2. faulty contraceptive because of sloppy manufacture.
3. contraceptive failure within the advertised margin of error.
4. negligence because of unjustifiable ignorance.
5. recklessness: swept along by passion.
6. indifference (didn't care at the time)
7. deliberate decision to have pregnancy (totally voluntary).

NOTE: bodily autonomy guarantees only a limited right to abortion.

A. The principals

The appellant was Jane Roe (a pseudonym), who challenged the constitutionality of Texas Statutes Articles 1191, 1192, 1193,1194 and 1196 concerning abortion.  Her lawyer argued that the interference of the State of Texas in Roe’s private life was:

• a violation of the legitimate use of police powers of the state.
• a violation of Roe’s right to privacy, which had been established in  Griswold v. Connecticut.  This was tantamount to violating her right to  due process and equal protection under the law (14th Amendment).

The appellee was the State of Texas.  It argued that:

• the anti-abortion statutes represent a legitimate exercise of the  police power of the state
• the fetus is a person and as such is protected under these laws.

B. The Texas anti-abortion statutes

When called upon judging the constitutionality of a statute, courts must interpret it.  They try to do this in two ways:

1. Finding the clear meaning of the statute.

Problem: Some statutes are passed with language which is  intentionally vague in order to satisfy different legislators.
2. Determining the legislative intent, i.e. to what end the legislature  enacted the statutes.

Problem: Legislators may have had a number of different goals in mind  in voting for statutes.  Moreover, in many states there are few records of the old debates over statutes.

1. These laws may be the product of Victorian social concern to discourage illicit sex. (This reason was not used by Texas, but it is a possible one.)

Response: This is not the proper concern of the state.
 
2. These laws may reflect the state’s interest in protecting citizens from unsafe medical procedures.

Response: Even if this was the purpose of the laws, today abortions performed with appropriate medical supervision are safe.
 
3. These laws reflect the state’s interest in protection of prenatal life.    Some argue that the fetus is a person from conception and therefore  the state has a right to prohibit killing fetuses (persons).  Others do not make this claim but claim that the state has an interest  in the protection of potential human life.

NOTE: the Court says "human life," but the issue, it seems, is potential personhood.
Responses:
•  There is some scholarly support for the claim that 19th century laws  were not designed to protect prenatal life.
•  If the laws were designed to protect prenatal life because it is a  person, the laws do not make sense.  If the fetus is a person, then why  is the physician (or person performing the abortion) not guilty of  murder? Why is the mother not an accomplice to murder.  Why is  attempted abortion not attempted murder?

The argument of the Court rests on the idea that there are three important interests at stake.
 

1. 1. The woman's interests

The woman has an interest in (a right to) privacy. (This was  established in Griswold v. Conn. and in other cases).  She has a right to liberty. She has a right to due process and equal protection under the law (14th Amendment).
NOTE: However, the Court argued that no rights or interests are  absolute.  The issue, then, was to determine when other interests are  sufficiently strong to override the woman's interest in privacy and  liberty.
 
2. The interest of the State in Health and Safety of Citizens.

The Court recognized the interest of the state to protect the health of  its citizens by regulating the practice of medicine.
 
3. The interests of the State in protecting potential human life.

The court recognized the interests of the state in protecting potential  human life.
 NOTE:
• The Court determined that a fetus is not a person under the  Constitution.  The Court revised the use of the word "person" in the  Constitution and in other legislation and never found it used as a  reference to the unborn. It concluded that fetuses legally have not been  considered persons.
• With respect to the claim that human personhood begins at conception  (and therefore legally we should include fetuses as persons) the Court  reviewed the controversy over personhood. It concluded that the  woman's important right to privacy and liberty could not be overridden  by the adoption of a controversial theory of the beginning of  personhood.

D. Conflicting interests and the stages of pregnancy

1. First trimester

Since abortions at this early stage are extremely safe, the state has no  compelling interest to regulate beyond its normal interest in  protecting health.  Because of the nature of the fetus at this early  stage, the State's interest in protecting potential life is also very  minimal.  Consequently, these other interests are not sufficiently  strong to interfere in the privacy and liberty of the woman.

2. Second trimester

Since abortions become more risky at this stage, the state's interest in  regulating them is stronger.  Hence, it can regulate, provided the  regulation is reasonably related to the health of the woman.  The  state's interest in protecting potential human life is also stronger,  since the potential human is closer to becoming an actual human  person.  However, that interest is not yet sufficiently strong to allow  the state to interfere in the woman's privacy and liberty.

3. Third trimester

The woman's interest in privacy and liberty are as strong as before.  However, once the fetus is viable, the Court believes that the State  may choose to regulate, even prohibit, abortion, as long as the life and  health of the mother are not threatened.

E. The decision of the Court
 

• Laws prohibiting abortions except those intended to save the life of the mother without regard for the stage of pregnancy and other  interests, violate the Due Process Clause of the 14th Amendment, and  are therefore unconstitutional.
• First trimester: no regulation is allowed during this period of time.  The decision is  left to the woman and a qualified physician.  However, the state may  regulate the medical practice of abortion.
• Second trimester: the State may regulate abortion in ways reasonably related to the  health of the woman.
• Third trimester (from the point of viability): the State may regulate, even prohibit, abortion, consistently with its interest in the potentiality of human life, except where necessary in  appropriate medical judgment for preservation of the life or health of  the woman.

1. There is a potential conflict between the third trimester and the viability requirements, since technology  pushes the momet of viability earlier and earlier.
2. In a related decision, the Court made clear that by "health" it meant both physical and psychological health.   Opponents claim that this amounts to allowing abortion on demand.
3. Planned Parenthood v. Casey (1992)
• The Pennsylvania legislature amended its abortion control law in 1988 and 1989. Among the new provisions, the law required informed consent anda 24 hour waiting period prior to the procedure. A minor seeking an abortion required the consent of one parent (the law allows for a judicial bypass procedure). A married woman seeking an abortion had to indicate that she notified her husband of her intention to abort the fetus. These provisions were challenged by several abortion clinics and physicians. A federal appeals court upheld all the provisions except for the husband notification requirement.
• The Constitutional Question is whether the amendments to the Pennsylvania law violate a woman's right to abortion as guaranteed by Roe v. Wade. In 1992, in a bitter, 5-to-4 decision, the Court again reaffirmed Roe, but it upheld most of the Pennsylvania provisions by imposing a new standard, namely, whether a state abortion regulation has the purpose or effect of imposing an "undue burden," which is defined as a "substantial obstacle in the path of a woman seeking an abortion before the fetus attains viability." Under this standard, the only provision to fail the undue-burden test was the husband notification requirement.