1. We observe that a value in something usually does not change as a result of a person's disposition toward it.
It is difficult to gauge the initial plausibility of this first premise. There are certainly moral relativists, nihilists, and perspectivists who will question or altogether deny this claim, all with different reasons and degrees. The moral objectivist, however, will likely be persuaded that (1) is true. Consider the value of oxygen. Even assuming that a person is distraught, or deluded, and wishes to no longer inhale the oxygen that keeps him or her alive, this doesn't at all imply that the oxygen no longer has value. In fact, most of us would say this person needs help.
2. If the loss of something brings about mourning, that thing has value.
I would wager that all of us know what this is like. Whether we have lost a loved one, a thing we hold dear for sentimental reasons, or whatever it may be, we're only sad about it because the thing that has been lost has value.
3. The unexpected termination of a pregnancy results in mourning.
Consider an expecting mother's disposition toward her unborn baby. If she wishes to have the child, a miscarriage would be considered tragic. This, however, leaves us with the following conclusion:
4. Therefore, pregnancy has value.
If it is tragic for a woman to have a miscarriage, is it not also tragic whenever the pregnancy is terminated by an abortion? After all, the only difference is a person's (or persons') disposition toward the pregnancy. The value of a desired pregnancy doesn't change simply because of a change in attitude.
The moral relativist may well chime in by suggesting that a thing only has value for the person who gives value to it. What is interesting about this, though, is that the same does not apply to other things we regularly give value to: oxygen, food, clothing, education, health, and happiness. Why make abortion the exception?