There are different types of developing defects that cause a urinary tract obstruction, resulting in urine backing up and causing the ureters and kidneys to swell.
The swelling may be barely detectable or very noticeable.
This condition, especially when is affecting only one kidney, very rarely require immediate action.
Often the condition goes away without any treatment before birth or sometimes after.
If the condition is still present months after birth, the baby is usually given antibiotic therapy till the surgery isn't performed to solve the defect.

Best wishes!