http://www.answersingenesis.org/docs2004/1116abortion.asp In short, this article from AiG describes how our bodies make killer T cells to attack foreign objects in our bodies. This is why in transplanting organs, the body rejects the organs. This article describes how the same exact process occurs during human pregnancy - and the there is a mechanism called IDO that actively fights the killer T cells at the Baby/mother interface. This goes against the common "Woman's right to choose" argument is that it is the "mother's body" and she should be able to do with it what she will. However, this science conclusively shows that the child growing inside a mother is distinctly foreign and unique - an individual person from conception, not an extension of the mother's own body that can be discarded.