For a long time now, the pro-life side has been working hard to prove with science and data that a pre-born human is still a human, and thus should have rights. However, in my experience it is now the pro-infantcide side of the abortion debate that clings to mysticism as valid grounds for legalizing abortion.
In other words, they cling on to some warped view of life contained in some pre-science era religious dogma (typically a really old Jewish outlook on human development). They use this as a figleaf to justify the legality of abortion - after all, if their "belief system" tells them pre-born life isn't human, how can abortion be illegal?
Hence, they can always throw a "religion" monkeywrench into the works - they make a point of making abortion a religious issue.
It IS a religious issue. Many animal rights activists think that all animals should have the same rights as humans. That's a belief system that contradicts the Judeo-Christian belief that humans are somehow special, set apart from all other living creatures, and thus entitled to a different set of legal protections.
Nobody is going to argue that an eight cell embryo isn't "human". But being "human" doesn't necessarily mean that it should be treated the same as a breathing baby or an adult person. The laws and social norms that have developed in human society were based on characteristics of humans who were born alive -- consciousness, ability to feel pain and fear, capacity for awareness of self in relation to others, etc. It is a huge jump to assert that those laws and social norms should be applied to early stage embryos which lack any of those characteristics, and a jump that can only be made on the basis of religious belief of some sort.
Likewise, some people oppose the death penalty, no matter how heinous the crime, and that is a position which is also based on religious belief of some sort, even though many anti-death penalty activists loudly protest that label. Personally, I don't ascribe any special status to humans. Generally, they are more thinking, aware, and have more long term perspective than any other species I'm aware of, but when creatures that are technically human don't have any of the characteristics that have been the basis for setting them apart from other animals, I don't see the need to treat them differently from other animals. If you see a pit bull viciously attacking a child, shoot it; if you see a human viciously attacking a child, shoot it. Both attackers are equally lacking in the characteristics that form the basis for human society and laws.
If a human embryo isn't wanted by its mother, and there are already millions of human children lacking decent homes and many lacking even food, then I see nothing wrong with allowing the mother to choose to euthanize it at a stage when it has no capacity for self-awareness. Before the third trimester, it has fewer human characteristics than a cat or dog.