The most effective way to argue that abortion is wrong is to argue that a fetus is a human life.
To do this, you need to argue that a fetus is not just some bunch of cells inside a woman. Instead, you must argue, it is a real human being. Once you have made this argument, the rest of the argument is simple. It is never acceptable to kill a human being who has not done anything wrong (with the possible exception of war). We do not kill people because their existence is inconvenient to us. We do not even kill people because they might (accidentally) kill us. We certainly do not kill them because they came into existence through rape or incest.