Kant would be against abortion as it couldn't possibly be univerisalisable. The maxim, "should I have an abortion," can't work because if everyone had an abortion there would be no more births and the human race would become extinct. It is also not be respecting people as ends in themselves as all humans are equally intrinsically valuable rational agents. It is also wrong as it uses the doctor as a means to an end as they are used to kill the foetus.
Kant himself never specifically says anything about abortion or when life beings. We can debate whether the foetus counts as 'humanity' or not. The foetus can't yet reason and may not class as rational agent. However, if the foetus is allowed to live, they can then use their reasoning to carry out their duties.
In the case of an ectopic pregancy, the foetus will die even if the mother chooses to continue with her pregnancy. An abortion could save the mother's life. Kantians may allow an abortion to take place in this case as no law could prevent the foetus from being born and they couldn't become a rational agent anyway.
Bentham would say that if an abortion produces the greatest good for the greatest number it would be the right thing to do. If the abortion supports his motavation, seek pleasure and avoid pain, it would be right as it follows human nature. Abortion isn't intrincially right or wrong itself.
You would need to use the Hedonic Calulus to determine whether the consquences of the abortion would make it right to do in that circumstance. If the consquence promotes the best of the 7 parts, (for example the more intense the better), the abortion would be the right thing to do.
If a teenage mother acidently becomes pregnant and she and family don't want the foetus, it would be okay to abort the child. It would produce the greatest happiness for the greatest number as the mother can continue with her education leading to other pleasures (e.g. a good job, pleasing her family and passing exams) and the extent of the pleasure would be bigger as the family would be happy without the child.
John Stuart Mill
Mill would say that abortion should promote the greatest happiness for the greatest number in order for it too be right. There is no general rule that does or doesn't permit us to perform abortion. Although there may be a general rule to not kill the innocent but this rule can be broken if breaking the rule promotes the greatest happiness for the greatest number. However, abortion and killing may go against Mill's harm principle.
Mill would also think that it's better not to abort as family is a higher pleasure. We should be aiming for these higher pleasures rather than the lower ones such as sex. However we need to still have sex inorder to have a family so lower pleasures are sometimes needed to reach the higher ones.
Aristotle beileved that life began at the moment of 'animation' which was ensolument. He claimed that this was 40 days for a male and 90 days for a female. This is when the foetus canb start to be recognised as a human rather than just looking like a ball of cells. However the foetus did have all of it's genetic features from the moment of conception and it's still only a potiental human at this point!
"The line between lawful and unlawful abortion will be marked by the fact of having sensation and being alive."
Artistotle believed that as long as the foetus wasn't alive, they could be aborted. However after ensolument abortions are not longer right as the foetus is alive.
Judith Jarvis Thompson
Thompson was against the idea that the foetus has a right to use the mother's body. She wasn't necessarily against the idea that the foetus has a right to life. She uses her example of the famous violinst to explain her views.
You wake up one morning and find yourself attached to a famous, unconscious violinist. He has a fatal kidney problem and needs to stay plugged in to you for 9 months inorder to survive or else he will die.
Thompson would say that you could unplug yourself from the violinist even if it resulted in death. The violinst has no right to use your body, although it would be kind to let him do so , you have no duty to let him. Similarly, the foetus has no right to use the mother's body. Carrying the foetus to full term is beyond her moral obilgations.
Singer would say that if the mother has a preference to not have the child, abortion would be the right thing to do. The foetus doesn't yet have a preference so they are not included in the decision of abortion. Only the mother and those effected by this decision are able to put forward their own preferences. The right and wrongness of an abortion depends on the preference of people effected by the choice.
However some would say that everyone has a preference to live. This is why humans are naturally unable to strangle or drown themselves. At some pointthe foetus will also be able to feel pain. Surely they have a preference not to feel pain or to die?
Singer would probably say that life begins when you have a preference. This way you are able to make decisions in your life. He thinks that you become a person once you are accecpted by society. This means that servely handicaped foetuses may not count as people if we can't except them.