It's not known exactly what causes undescended testicles, but they are thought to occur if something disrupts the normal development of the testicles.

Normally, once the testicles have formed, they stay inside the baby’s abdomen (tummy) until about two months before birth. At this point, they start to gradually descend from the abdomen into the scrotum, finally settling in the scrotum shortly before birth.

The descent of the testicles is a complex process that is influenced by a number of factors, including certain hormones and the pressure within the abdomen. If there are any problems affecting these, there is a risk of the testicle(s) not descending.

Most boys born with undescended testicles are otherwise completely healthy and no reason for their condition is identified.

Who's most at risk?

It is not known exactly why some boys are born with undescended testicles, but several things have been identified that increase the risk of a baby boy having the problem. These include:

• Low birth weight – boys with a birth weight of less than 2.5 kg (5.5lb) are more likely to be born with undescended testicles than those with a normal birth weight.
• Being born prematurely – the earlier a boy is born, the more likely he will be born with undescended testicles. Read more about premature labour and birth.
• Having a family history of undescended testicles – having an older brother with undescended testicles means that a boy is more likely to be born with the condition compared with the general population.