It's a moral position.
Being pro-childbirth shouldn't be about being conservative or liberal. Nor should it be about being Christian or some other specific belief. It's about being a decent human being.
Let me explain. There is a child inside your body. If you really didn't want to raise a child (the "choice" to not have a baby), you should not have messed around with any men. Period. Likewise, men shouldn't be messing around with women if they don't want children. Keep it in your pants. But now that you didn't make responsible choices to begin with, I would expect you to be responsible enough to carry the child to term and offer it up for adoption and not take away its choice. In other words, being for abortion means you are not a decent human being but rather a selfish and narcissistic *******, who would prefer to kill others who are smaller and weaker than you, rather than have them get in the way of your singles lifestyle.
Whether you are an atheist or Christian, left or right politically, nobody should be doing this. It's barbaric. In fact, just how barbaric is it? In ancient Judaism, they specifically forbid the sacrifice of children to the god Molech. This is prehistoric Israel, and even they are more civilized than this.