“We know guidelines recommend surgery by 18 months, but studies show many boys are not operated on by that time. In our study half were later, in others even more,” he said. The doctors taking care of babies need to be more aware of the guidelines, he said, and parents should expect their sons to be fully examined at every checkup.

“If it doesn’t come down by 6 months, it’s not going to come down on its own,” said Dr. Thomas F. Kolon, a pediatric urologist at Children’s Hospital of Philadelphia and professor of urology at the University of Pennsylvania School of Medicine. He is the lead author on the American Urological Association guidelines for the evaluation and treatment of cryptorchidism. “Start thinking of therapy by 6 months and really do something by 18 months.”

In general, he said, the boys with unilateral cryptorchidism — one testis descends, the other doesn’t — do very well, but “the bilaterals take more of a hit.”

[Related: When parents worry that a baby’s penis is too small.]

Although the risk of testicular cancer is higher in boys with cryptorchidism, it still remains a rare disease; to monitor themselves, adolescents at risk should be taught to do a monthly testicular self-exam, looking for changes that might suggest cancer, Dr. Kolon said.

The surgery, called orchiopexy, is done under general anesthesia and is more complicated if the testes are higher up. If they are up above the scrotum but still palpable, the surgeon will make a small incision in the groin, free the testis, and then another small incision in the scrotum, where it will be anchored in place. If the testis cannot be felt at all, the surgeons may do a laparoscopic procedure, putting a camera in through the baby’s belly button, and if they find the testes in the abdomen, there may be a two-stage operation, first freeing up the testis and then, six months later, in another procedure, bringing it down into the scrotum.

In some newborns, the absence of testes in the scrotum is part of a larger and sometimes more serious syndrome, and those children may need additional medical attention.

On the other hand, many of the baby boys that pediatricians refer to urology because of undescended testicles actually turn out to have what are called retractile testes, which do not need surgery, said Dr. Aseem R. Shukla, a pediatric urologist at the Children’s Hospital of Philadelphia and an associate professor of urology at the University of Pennsylvania School of Medicine. “If you’re in a cold room, the pediatrician’s hands are cold, the baby very ticklish,” he said, then the cremaster muscle, which connects to the cord of the testicle, raises it up.