By means of the recent sophisticated technology regarding prenatal diagnosis, congenital mesoblastic nephroma (CMN) has become detectable before birth, or at a younger age than previously. Recently we treated an infant with a huge CMN in whom fetal asphyxia and tumor rupture occurred during the perinatal observation period after prenatal detection. Emergency surgery was required, and the postoperative course was complicated. The treatment strategy of the perinatal care team should focus on (1) reliable maternal transportation (2) continuous monitoring of fetal condition including cardiovascular status, (3) control of polyhydramnios to avoid premature labor, and (4) elective surgery at a stable or stabilized condition. Emergency surgery should be performed when circulatory disturbance, respiratory distress, and/or impending rupture are suspected.