During the baby’s development, if the testicles are large on one side and small on the other, there is usually no need to worry too much, it is a normal phenomenon. However, if the size difference is more obvious, we should consider going to the hospital for consultation and treatment according to the cause. The common causes of a baby’s testicles being big on one side and small on the other include the following: 1. syringomyelia: in normal baby boys, there is a pouch in the scrotum called the “peritoneal sphincter” that should close on its own and become a very thin fibrous cord, a small part of which is outside the testicle to form the intrinsic sheath, but this pouch will close with the abdominal cavity if not at birth. However, if this band is not closed at birth, it will be connected to a very small channel in the abdominal cavity, thus allowing abdominal fluid to flow into the pouch and, together with scrotal secretions, form a syringomyelia, causing the scrotum to swell up. However, when the pressure of the syringomyelia is high, or if the child has not healed on its own after the age of 2, surgery is required in a timely manner. The risk of surgery is not too great, and it usually needs to be performed as early as possible, otherwise it may affect the development of the testicles. 2. Hernia: If a child is born with an enlarged side, it is more likely to be considered congenital. Due to incomplete closure of the peritoneal sheath at the groin at birth, the small intestine in the abdominal cavity enters this sheath and a hernia is formed. Regardless of which kind of hernia is difficult to heal on its own, usually more than one year old should be treated surgically, preferably from 8 months to 1 year old is best, generally do not exceed 2 years old. 3, testicular dysplasia: congenital testicular dysplasia is mostly caused by genetic factors – chromosomal abnormalities. Acquired testicular dysplasia means that the patient’s chromosomes are normal, but the testicles develop abnormally due to the influence of various pathogenic factors during the fetal period, childhood or early childhood. At present, there is no good way to treat testicular dysplasia. 4.Cryptorchidism: The fetus itself has cryptorchidism, and after the newborn is born, the testicles do not descend into the scrotum, and there are unilateral and bilateral cryptorchidism. The location of cryptorchid will seriously affect the development of testicles, and the higher the location, the worse the development, especially if the cryptorchid is located above the groin, it needs to be operated as soon as possible, otherwise it will probably affect the child’s future reproductive function.