Is the kidney just enlarged or have they found a reason it is enlarged?
DS2 has/had a right multicystic dysplastic kidney which was discovered at my 20 week scan. I was then scanned every 4 weeks until DS was born at 38 weeks weighing in at 10lb. He was given anitbiotics from birth and then tested for reflux at around 6 weeks old, all was deemed fine so they just kept an eye on him.
During the pregnancy his kidney became increasingly enlarged with more cysts on it and then after birth shrivelled up and 'disappeared', so he only has one kidney.
One kidney can do 90% or more of the work that two can. DS is now 10 and leads a normal life. It has never been suggested he restrict it in any way. He does love telling everyone he only has one kidney though...
This is exactly what happened to my goddaughter. I dont know what the condition behind it was but one of her kidneys was way over large. She was on antibiotics for quite some time after birth but is now a very lively normal 4 year old. Absolutely no other health issues and no outward sign of kidney issue.