If this is the case then why if I hit a pregnant woman in a car and killed her would I go to prison for manslaughter against her and the baby? If it is not a person yet it should be insignificant that I killed it. The fact that the mother wanted the baby is irrelevent.
As explained by a friend of mine, in words better than I could do myself:
The fact that the mother wants to carry to term is the only thing that is relevant.
"Why? Because in the law there is a provision for protection of potential items and revenue, if you can reasonably show that the potential item or revenue would have become a complete item or real revenue had it not been interfered with. The value of the item for the purposes of the damages done is extrapolated based on what is a reasonable assumption for what its value might have been. The amount of revenue considered lost is based on reasonably expected earnings.
For example, if I started writing a story and then someone came along and took it while it is still unfinished and published it (in its unfinished form), I could sue them for damages of lost income based on what I might reasonably have earned were it to be completed and published by me. The value for this potential income calculated is determined by the judge based on a number of factors. Whatever value the judge calculates for me, it would be a hell of a lot higher if the author who was robbed was, say, J. K. Rowling, instead of me, simply because even if I write the next Lord of the Rings, I could probably never hope to make as much as Rowling would if she sneezed in a napkin and published the smear.
The same logic extends to other things, like if I was building a house or a car and you came along and destroyed it before it was complete, it would count as destruction of property, not destruction of potential property, and I could sue you for damages based on the potential value of the item, not simply the cost of its parts. In fact, if I bought a custom car from someone that I intended to use to make money with, and it was being delivered to me and I had not paid for it yet, and you destroyed it, I could sue you for the money I was unable to make even though I hadn't bought the car yet (but I couldn't sue you for the value of the car itself, because I had not paid for it). There is lots of tricky legal crap involved with proving your claim on the (potential) item and the reasonable expectancy of the money you might make, and thus, your rights to sue... but once that is established, it's suing' time.
So when you kill the fetus of a mother who was planning to carry it to term, you have literally destroyed their potential daughter. Yes, you did not kill a human being, but the potential value of what you stole from that woman was equal to the value of a human, because if the mother does intend to have the child, chances are very, very high nowadays that she will. In theory, without the fetal homicide clause, the mother could sue you for the expected value of what you took from her, just as with anything else, be it potential story royalties or a potential car. But that just gets wonky because what is the value of a human being - a child, someone's offspring? You can't put a realistic price tag on that, so, instead, they do the next best thing... they say the expected value of a wanted fetus is a child, and you have taken that potential value away from the mother, which is equivalent to the a priori murder of their child. Thus, fetal homicide.
So why is this different from abortion? The answer is simple. The expected value of a wanted fetus is the value of a human child, because a wanted fetus will - under reasonable assumptions - be a human being eventually. The expected value of an unwanted fetus is... nothing... because the fetus will not become a human being (under reasonable assumptions). Therefore, the abortion doctor that performs the procedure is not killing a potential human being... because the fetus doesn't have that potential from the moment the mother says, "I want an abortion."