The problem with this question is that reconciliation implies two sides compromising or at least coming to a peace with one another. This does not happen between the Kowalskis and the Dubois in A Streetcar Named Desire, nor is it even suggested.
In the original play, the child is not really a symbol of reconciliation. If anything, the child will keep Stella in her abusive relationship with Stanley. It is never said openly, but when Eunice tells Stella "Don't ever believe it. Life has got to go on. No matter what happens, you've got to keep on going," she might also have the baby in mind. Stanley is an unrepentant brute, and it is unlikely that the baby is going to change him—at least, not according to any signals in the play's staging or dialogue.
About as close to reconciliation as the baby might get the two families is in the socio-political symbolism of the Kowalskis (the New South) and the Dubois (the Old South). Kowalski's marrying and having a child with Stella reflects how the Old...
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