NCI: A type of kidney tumor that is usually found before birth by ultrasound or within the first 3 months of life. It contains fibroblastic cells (connective tissue cells), and may spread to the other kidney or to nearby tissue. Congenital mesoblastic nephroma is more common in males.,NCI: A congenital malignant neoplasm of the kidney characterized by the presence of fibroblastic cells.,NCI: A low grade childhood congenital malignant neoplasm arising from the kidney. It is characterized by the presence of fibroblastic cells. The majority of cases occur in the first year of life. Complete excision is usually associated with an excellent prognosis.