The University of Virginia School of Medicine explains that when the testes descend, they are pulled from the abdomen to the scrotum by a piece of connective tissue known as the gubernaculum testis. This generally occurs around the time a male baby is born. However, babies are occasionally born with one testicle still undescended. In most of these cases, the second testicle descends in the following months.
The University of Virginia School of Medicine states that the testes first form before the third month of pregnancy near the lumbar area of the back. Only after that do hormonal and growth processes move them downward. A similar process occurs in the ovaries, but the testes are moved further. The gubernaculum testis that aids this process is formed in the seventh week of pregnancy. It is attached at one end in the scrotum and at the other to the testis themselves. Over time, the gubernaculum testis shortens, pulling the testes and their ducts to a muscular opening known as the inguinal canal. This first occurs between the third and seventh month of pregnancy. The testes remain in the region of the inguinal canal until around the time of birth, when androgens cause them to descend the rest of the way.