Praise be to Allah.
The basic principle is that he does not have to give a gift to the wife who has not given birth; if she later gives birth, he does not have to give a gift to the other wife. Fairness means that he should give them gifts of equal value, i.e., if he gives the first wife one hundred when she gives birth, he should give his other wife one hundred when she gives birth. He does not have to give her anything before she gives birth, but if he is afraid that problems may arise, and he wants to give a gift to both wives each time one of them gives birth in order to avoid problems, then this is fine and is regarded as kind treatment in the interests of harmony. And Allaah knows best.