Fetuses are human, and are biologically and genetically separate from their mother, meaning they should not be treated the same as a limb or organ (i.e., they are not part of the mother's body). The mother doesn't have a medical proxy relationship to the fetus, since a proxy cannot order the death of his/her charge without medical cause, whereas a mother can do so. This indicates a form of ownership. This also conflicts with the notion of unalienable human rights. You could debate personhood, but legally this is tricky territory since many states have a law that protects unborn children (Unborn Victims of Violence Act), as does the U.N. The UVVA states that a fetus that is killed as a result of assault on the mother will be counted as a victim of murder, despite the legality of abortion. This seems to indicate that the law currently does not have an objective or standard definition of personhood. Morally, we can recognize that all humans should have an equal and inherent right to life (and liberty and property/pursuit of happiness, etc.). At least this is what was established during the Civil Rights movement. But abortion is exempt for an unjustified reason. So in order to justify abortion as morally acceptable, it seems reasonable to say one has to consider an arbitrary, ill-defined definition of personhood as superior to the scientifically-based definition of human. A fetus is not a person, one would say, and is therefore not eligible for any rights or protection from the law when the mother chooses. Interestingly, using those UVVA edicts, a fetus does in fact have those rights inherently. When a mother exercises her right to an abortion, she waives those rights. There is no legal precedence of this, except in past situations of slavery and patriarchal ownership of wives by husbands. TL;DR: Fetuses are human and our constitution declares "certain unalienable rights, among these are life, liberty, and the pursuit of happiness". Abortion undermines the most basic rights that our country is founded upon.