Newborns with large and small testicles may have congenital malformations, testicular syringomyelia, inguinal hernia, cryptorchidism and other causes. 1. Congenital developmental abnormality: newborns in the mother’s fetal period because of genetic anomalies and the development of testicles on both sides of the asynchrony, so the testicles will be big on one side and small on the other. 2. Testicular syringomyelia: the amount of fluid in the testicular syringomyelia cavity exceeds the normal range, resulting in the enlargement of the scrotum, and one side of the testicle is compressed, so it looks relatively small. 3. Inguinal hernia: Inguinal hernia can enter the scrotum through the weak part of the superficial inguinal ring, and press one side of the testicle and appear this symptom. 4. Cryptorchidism: the descent of one side of the testis is incomplete and does not enter the scrotum due to the short spermatic cord vessels and dysfunction of the testicular traction band, so the symptom appears. Newborns with large balls on one side and small balls on the other side may be related to a variety of factors, and it is recommended to consult a doctor for a clear diagnosis.