Recently, our pediatric surgery department successfully performed a retroperitoneal teratoma removal surgery for a newborn baby boy who was born only 9 days ago. The baby weighed 8 pounds at birth, and the teratoma cut off weighed 1.5 pounds, accounting for nearly 1/5 of his body weight. This little patient has now recovered and discharged from the hospital. On November 13, Mao Mao was born in the obstetrics department of our hospital. The doctor found that Mao Mao’s little belly was bulging very large, and immediately asked the deputy director of pediatric surgery, Zhou Xuewu, to make a consultation. The ultrasound examination showed that the right abdominal mixed mass was 10cm×7cm×10cm in size, which was considered to be a huge teratoma, and was then admitted to the pediatric surgery department for inpatient treatment. After Mao Mao was admitted to the pediatric surgery department to improve the relevant examinations, Zhou Xuewu thought that the best treatment for Mao Mao was to undergo surgical resection. After sufficient preoperative preparation, on November 22, the 9th day of Mao Mao’s birth, under the anesthesia of Zhou Hai, deputy director of the Department of Anesthesiology, Zhou Xuewu’s medical team performed the left retroperitoneal giant teratoma resection for Mao Mao. Mao’s belly was just the size of a volleyball; and the tumor in his stomach occupied almost most of the abdomen, squeezing the intestines to the right, the spleen, pancreas and left kidney to the pelvis, and the tumor invaded the esophagus and gastric fundus. Zhou Xuewu’s medical team chose the smallest surgical instruments, carefully separated the tissues around the tumor, disconnected the blood vessels supplying the tumor tissues, removed part of the esophagus and gastric fundus and made repairs. After more than an hour, the “meatball” in Mao Mao’s stomach was successfully peeled off and removed. The tumor weighs about 1.5 pounds, accounting for about 1/5 of the child’s weight. Deputy Director Zhou Xuewu explained that teratoma is an irregular growth and development of early human embryonic cell groups, with a benign rate of 90%, but it should be removed as soon as possible after birth, otherwise the chances of it turning malignant will gradually increase with age. Simply put, a teratoma is formed when some embryonic stem cells differentiate abnormally locally during the development of embryonic tissue, and it may try to differentiate into a sibling of Mao, but it fails to do so. Because the differentiation of teratoma cells in the embryonic state is extremely inconsistent, it often includes undifferentiated cells, mature cells and formed organs within a single tumor. However, it is rare for a child this young to have such a large teratoma.