While I understand your reasoning, I don't agree that it's a good enough reason not to ban abortion. We don't pass laws to ban things just because we believe the laws are preventative. Most of the time, they aren't. We have laws against stealing yet robbery is common. We have the death penalty against murder but we have a horrible epidemic of murder going on in various places in the world. Laws aren't just passed to prevent things, they are sometimes passed to address and punish things they can't stop because those things are simply wrong.