When I see boys for routine physicals, I always check their testicles for lumps or swelling. The most common problem I find in the first year of life is something called a hydrocele. A hydrocele is a collection of fluid around the testicle.

Hydroceles have an interesting history. Although a baby’s gender is decided at the moment of conception, both sexes develop along the same path until the third month of pregnancy. At that point, a group of cells in the lower abdomen develop into ovaries in females or testicles in males. If the baby is a girl, the ovaries are in the correct anatomic position. If the baby is a boy, the testicles need to descend into the scrotum. This process takes about six months. As the testicles move downward, they pass through the inguinal canal before reaching their proper position in the scrotum.

After the testicle enters the scrotum, the inguinal canal is supposed to fuse thereby separating the abdominal cavity from the scrotum. In some cases, this doesn’t happen and a small amount of fluid travels into the scrotum. Most of the time, this occurs before birth and the hydrocele will be detected at the baby’s first examination. In some cases, fluid moves into the scrotum after birth, in which case the hydrocele won’t be found until the baby is older. Most turn up between 1 and 4 months of age.

Hydroceles are not dangerous and usually resolve before a child’s first birthday. If they persist beyond a year, there’s a possibility the child also has an inguinal hernia. In this case, he should see a pediatric urologist or a general pediatric surgeon.