Some people would argue that the child will not have a very good life if he or she is born to parent(s) who did not really want him or her in the first place and that the parents may not be able to support a child emotionally and physically. They might also argue, particularly in the case of teenage parents, that having a baby can ruin the life of the person who has the baby.
Sometimes people argue for abortion due to medical reasons, either because having a baby and being pregnant is putting the mother's life at risk, or because tests have shown that the baby has something wrong with him or her
A major argument for abortion is in cases where the mother has been raped and has ended up being pregnant