The problem in the anti-abortion approach is that you want to equal potential with what is real in present time. Is like saying an acorn is the same as a tree. Both are parts of a life cycle but while we happily throw acorns away, people give a very different consideration and value to cutting a100 year oak. The acorn is a potential tree with all the 'oakness' of the tree but is not a tree, so we do not treat it as such. You do not harm a 100 year old oak tree to save an acorn. Apply it to the human embryo.

People often put pine seeds in salads. These pine seeds, if left by themselves can become many trees, a real pine forest which create a habitat for many other creatures. However, nobody will equal a bunch of seed in my salad with the whole ecosystem although the potential is there. A $100 bill in the stock market has the potential to become $1 million, however, put to choose nobody will choose a $100 bill even already invested in an optimum way in the market over 1 million dollars in stock value. An unrealized potential is not a done deal so it has less value than an already realized potential. Many people laugh when somebody throws eggs at a politician but nobody has had the gall to throw little chicks at anybody. They know nobody will approve. People know they are not the same. It is ludicrous to have the unrealized potential rights to supersede those of the realized entity.

An embryo does not have sensations or sentience as it does not have a nervous system and a brain. An embryo is not an independent socialized organism, something that is necessary to be a full human being. An embryo is part of the mother body in the best of cases or it can be considered a parasite in the worst case as it uses the resources of the mother body to survive. Giving all this, the mother should be able to refuse to share her resources (as the body is the ultimate private property) with the embryo and will not be killing a human person but an embryo.

Once this is clear, one can understand that the origin of that anti-abortion position comes from thinking that there is something outside sentience and biology that makes us human and that ‘essence’, is what is called a ‘soul’’ even though they avoid that word as much as they can in an argument. A soul is a religious concept because its existence is based on faith, it cannot be proved objectively.

On the other and more matter of fact hand, there are stages of development in humans and we all agree they do not have the same rights. A toddler cannot go by her/ himself to school no matter how much he/ she might want to. One parent has to take him. The toddler cannot choose what religion to join, what to eat or where to live among many other things. A person younger than 18 cannot give consent for sex, and his/her parents decide where he/she studies, where he lives, and many other things. A person younger than 21 cannot decide to drink alcohol. It is easy to see that the younger a human is the fewer rights he has and the more parents decide for him. Why is that? Because we all recognize that before 18, a human is not ready to make some decisions and parents are trusted to take the best decisions for them. A pre-person like an embryo simply fits into this scale. It is not fit to make decisions even about their own life and the mother is the one who should decide to carry the pregnancy to the end it based on her and the pre-human best interests. (Bringing an unwanted child to the world to be rejected, to suffer and maybe be mistreated is worse than aborting, for both the fetus and the family in my opinion)

Do pro-lifers count their age from conception? That would suck.

You are derived from an egg first created while your own mother was in the womb, but of course that would be a completely arbitrary date, unlike your birthday.