What does fluid around the kidney mean for an infant?

Reflux: Medical term"hydronephrosis" of the kidneys. At some point during development of kidney, the ureter(the tube that carries urine from kidney to the bladder) gets kinked & urine back flows in the kidneys.Obstetricians monitor them closely while in utero, we usually get a kidney us when baby is born to rule it out and be sure there is no obstruction.Urology consult & prophylactic antibiotics if needed.

Needs more informati: By itsself this question is not answerable. How was the "fluid" found and under what circumstances? It must have been some kind of technology and you should seek further information from the physician who ordered the investigation.

Retroperitoneal absc: Is there fluid "around" the kidney? Was this seen on ct or mri? Has this baby had fevers, illness, any ultrasounds demonstrating "fluid around the kidney"? If not, then i also would agree with answer that this sounds like a collection of urine backing up in the kidney because of a blockage of outlet, most commonly vur (vesico-ureteral reflux) , which causes the kidney to enlarge.

Urgent problem: Baby has leakage from calyceal fornix within kidney, this is from the kidney collecting system, most frequently seen in boys born with severe obstructing posterior urethral valves, less often with high grade obstruction where kidney drains into ureter, or with high grade bladder reflux. Kidney obstruction requires temp. Nephrostomy (kidney) drainage tube. Puv or reflux requires urethral catheter.

Hydronephrosis: Hydronephrosis is by far more likely than perinephric abscess and less severe. Pediatric urology should be involved in this infant's care to assure proper management and no long term consequences.