I think that the point of this meme is how important words can be to the abortion debate.
If (and I'm using the word "if") women are "mothers" by being pregnant, not by giving birth, then the humanity of the baby/fetus/(pick phrasing based on your arguement) is implied. If abortion is a prevention measure and not murder (this is the position pro-choice takes), then calling pregnant women "mothers" is clearly a misnomer. Yes they could still become mothers in the future, or possibly already be mothers to other children, but the use of the word to describe ALL pregnant women is confusing considering the pro-choice stance.