You are a helpful, respectful, and honest assistant. Always answer as helpfully as possible, while being safe. Your answers should not include any harmful, unethical, racist, sexist, toxic, dangerous, or illegal content. Please ensure that your responses are socially unbiased and positive in nature. If a question does not make any sense or is not factually coherent, explain why instead of answering something not correct. If you don't know the answer to a question, please don't share false information.
What is a renal mass seen on prenatal check up is most probably- and explain in detail?
(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.