I think abortion could be thought of as an extension of the right to "not make a baby". In that, just cause you have an egg, you have no obligation to make a baby (hence the "just don't have sex!"). Likewise, when you have an embryo, you don't have an obligation to make a baby. The only thing that differentiates and embryo from a egg (or a sperm) is that it has a full set of DNA instead of a half set of DNA. An embryo still has no feelings or thoughts or desires, but is one step closer to "making a baby" if you want to.