When couldn't his first wife have a 'late' child at 50? It might be a nice play on Sarah who had Isaac in her old age.

Might it be easier to explain #'s of children over time by having the family have at least one set of multiples? We now know that fraternal twins can result from a genetic tendency to produce more than one viable egg in a cycle. Perhaps Rav Hisda's wife can come from a family where twins were common.