If both kidneys are affected the newborn should be started on an antibiotic immediatly after birth and have the kidneys tested after one month for maturity.
I'm not sure but the treatment depends on how badly the condition is and how dilated the kidney is. The kidney grows over time so they might not be able to do anything about it during pregnancy.
most of the time nothing will be done before the baby's birth unless the situation gets significantly worse, then the doctor may opt for the baby to have a surgery done while still in utero.