A stock argument for abortion centers on the criterion of personhood. Aborting the baby is justified because the baby is not yet a person.
For discussion purposes, let's grant that the baby is not yet a person. Now let's take some comparisons. There are operations which, in a sense, require the medical team to kill the patient. That sounds paradoxical. What I'm referring to are operations where the patient's body temperature is lowered to the point where he has no vital signs. Technically, he meets the criteria of clinical death. He's indistinguishable from a corpse–albeit a very fresh corpse!
Suppose, for the sake of argument, that we say a patient in that condition is no longer a person. Would it be murder to pull the plug? In a sense, he's already dead! So you're not even killing him.
But, of course, if he's reheated, he will come back to life. So, is it murder to pull the plug?
Take another comparison: some comatose patients are declared to be in a persistent vegetative state. Yet some comatose patients in that condition regain consciousness. They are normal again. Or even if they suffer some lingering physical or cognitive impairment, they are still persons.
Suppose, for the sake of argument, that we say that someone in a persistent vegetative state is no longer a person. But suppose we foreknew that in a week or month or year, our comatose patient would regain consciousness and be normal, or nearly normal. Would it be murder to pull the plug? Indeed, there are medically-induced comas. But the point is to bring them back out of the coma.
Well, if a baby is allowed to come to term, that's what happens. He becomes a person–even if (ex hypothesi) we deny his prenatal personhood.
Now perhaps someone would object that my comparisons oversimplify the issue. The comparisons are disanalogous, for the baby is dependent on its mother. To be analogous, the patient would have to be dependent on a second party. But is the second party obligated to be at the patient's disposal? Does he have that claim on a second party? To that I'd say to things:
i) That complication is a backdoor admission that the (alleged) absence of personhood is an insufficient criterion to justify abortion. There must be an additional warrant, such as an "undue burden".
ii) Apropos (ii), suppose your mother requires weekly blood transfusions to stay alive. Suppose she has a rare blood type. As luck would have it, you have the same blood type. Do you have an obligation to make yourself available to keep your mother alive?
Some people might bite the bullet and say, no, even your own mother doesn't have that claim on your freedom. You have the right to let her die, even though it was within your power to keep her alive by donating your blood.
If so, that illustrates the moral consequences of the abortion ethic. It boils down to radical autonomy and power. But if that's the bottom line, then whoever has more power has the right to kill whoever has less power in the pursuit of absolute freedom.
That's why laws are necessary. Some people have no conscience.