What does an enlarged kidney mean in a fetus?

Large kidney: Depends on the other details. The kidney could be malformed, there could be a tumor, the kidney could be obstructed on one side or if both kidneys are enlarged then much of the urinary tract is likley obstructed. I would need more specific information to give a more specific answer.

Several causes.: Trisomy 18/13, polycystic renal disease, multicystic dysplastic kidney disease, hydronephrosis, renal tumors (mesoblastic nephroma) are all possibilities. Expert mfm advice and serial sonography are required.