a renal mass seen on prenatal check up is most probably-
(Refer: Nelson’s Textbook of Pediatrics, SAE, 1st edition, pg no. 2467)
 
Prental renal tumours

Enlargement of a kidney on prenatal imaging is usually due to hydronephrosis or a cystic renal enlargement, such as multicystic dysplastic kidney or polycystic disease
Solid tumors of the kidneys are rarely seen in the antenatal period, but can be diagnosed by ultrasound.  Most develop properly as the baby is growing in the womb.
Wilm’stumor
Mesoblastic nephroma
Neuroblastoma
Renal sarcoma