I've been debating the issue of abortion on this forum and on others. There is one argument that I encountered that seems to be the strongest argument from the pro-life side. Here's an excerpt from a member on another forum:

This comes after I acknowledged that a woman's fetus is a human life...(convo took place on DebatingChristianity.com)
RightReason said:
Why? Like you said, if we recognize it for what it is – a human life, how can you say it has less value than a human outside of the womb, just because it isn’t yet out of the womb? That is suggesting the value of a human being is based on what he/she can do or “bring to the table” so to speak and not simply inherent in being a human being. It is precisely the kind of thinking that says grandpa is of less value than young strapping Johnny because he’s past his prime. It’s the same kind of mindset that says paraplegic Joe has less value than the football player Joe. That smarter people have greater value than someone with a lower IQ.

A similar point was made by conservative commentator, Ben Shapiro:
So the real question is, where do you draw the line? So are you going to draw the line at the heartbeat? Because it’s very hard to draw the line at the heartbeat because there are people who are adults who are alive because of a pacemaker, and they need some sort of outside force generating their heartbeat.

Okay, are you going to do it based on brain function? Well, what about people who are in a coma? Should we just kill them? Right, the problem is anytime you draw any line other than the inception of the child, you end up drawing a false line that can also be applied to people who are adults. So either human life has intrinsic value or it doesn’t. And I think we both agree that adult human life has intrinsic value. Can we start from that premise?

For debate:
1. What is your view on the above argument? Do you agree or disagree with it?
2. If you disagree, where do you draw the line? When does a human life have moral value or when does it not have it?
 
What is your view on the above argument? Do you agree or disagree with it?
The problem that I have with the pro life argument in the OP is that it's just an assertion without proof. How does the pro life side know that there aren't conditions or exceptions, like only sentient human life having moral value? I think of a person who is on artificial life support but is confirmed brain dead. Would it be murder to pull the plug on them? In a sense, a fetus is on artificial life support since it can not survive on its own in its early stages. The fetus doesn't even have major organs in its early stages.

Again, I'm open to accepting the pro life argument but without proof it leaves you just as open to attacks or is no different than the speculation put forth by the pro-abortion side when they bring up "personhood".

If you disagree, where do you draw the line? When does a human life have moral value or when does it not have it?
I draw the line with awareness and pain. I don't want any innocent life to go through pain and suffering, especially one as helpless as a child or even a developing fetus. There is no consensus on when simple awareness (as opposed to all of the loaded concepts that goes along with consciousness - thoughts, self-awareness, etc.) begins but the ability for fetal pain is thought to exist starting around 20 weeks of gestation. So, I'm against abortions after 20 weeks, and to be on the same side I'd even stop abortions after the first trimester.
 
A single cell is alive.

Clearly. And obviously, it’s human. I mean, it’s not another kind of animal. But it doesn’t yet have a nervous system. It’s potentially a person.

But special rights for the fetus?

And that’s really the position the pro-life position is taking. It’s as if we’re saying I have a duty to donate a kidney to someone, to be enforced by law if necessary.