This is a very good question. Stella's pregnancy may have several functions, such as being a very conspicuous symbol of the exceptionally powerful sexual relationship between her and Stanley and her total dependence on him for support; but the coming baby seems primarily intended to serve as a "ticking clock," to use a Hollywood term. Blanche has moved into their small apartment where she is always in the way. She is taking up a space that will have to be used for the baby when it comes. There will be a crib, a playpen, a bassinette, a dresser for baby clothes, a place to park a stroller, and a lot of the other paraphernalia required by babies. There is no other space for all this stuff except the area Blanche is currently using as a bedroom and dressing-room. Blanche is like a baby herself. She likes to be attended to by her sister Stella--but Stella will soon have a much more serious interest and responsibility than in an aging older sister. It is obvious that when the baby comes, Blanche will have to move out. But she has no money and no place to go. This will make her relationship with Mitch one of desperation. If she can get Mitch to marry her, that will solve all her problems. But otherwise she is facing the reality she dreads. No doubt her short future in the outside world would involve prostitution, acute alcoholism, and complete degradation. That was what was happening to her in her home town after she lost Belle Reve and what she ran away from. As it turns out, her problem is solved for her when she is taken to a state institution for the insane. The baby is born and the Kowalskis return to being a happy and healthy family with a future, while Blanche, a representative of the decadence of the Old South, is escorted off into oblivion.