During pregnancy, the testicles form inside a baby boy's abdomen (tummy), before slowly moving down into the scrotum about a month or two before birth.
It's not known exactly why some boys are born with undescended testicles. Most boys with the condition are otherwise completely healthy.
Being born prematurely (before the 37th week of pregnancy), having a low birth weight and having a family history of undescended testicles may increase the chances of a boy being born with undescended testicles.