In "Popular Mechanics," the baby could be said to represent the respective egos of the warring couple. It's abundantly clear right from the start that neither the man nor the woman has the best interests of the child at heart. They each want the baby, not because they love him, nor because they genuinely believe that they can give him the best upbringing, but because they don't want their partner to have him.
The satisfaction of ego has taken precedence over the child's welfare. The baby is not to be loved and valued for his own sake; instead, he is nothing more than a pawn in a titanic battle of egos between two selfish, emotionally needy people.
The biggest irony in the story comes in the chilling last line:
In this manner, the issue was decided.
Only the issue hasn't been decided at all, because neither party to the conflict has what they want. And the reason for this is that the child appears to be dead, killed due to being pulled about by the man and woman as they struggled to grab hold of him.
On another level, however, one could say that the issue has been decided, in that the egos of the warring couple have finally been satisfied. As neither party in the conflict has what they originally wanted, both have effectively saved face, but only at the expense of a baby's life.