I quickly want to stress this; the majority of people who are anti-abortion are religious, and it's because of the concept of a soul.
The big question for religious people is "When does the soul actually enter the body of the zygote to form a human?" In that case, we need to have a religious discussion.
Otherwise, it's purely ethical, and the answer should be related to science and facts/figures/statistics that we have on why people get abortions.