Undescended testicles are caused by something that interrupts the normal development of the testicles.
Genital development
In normal development, once the testicles have formed they stay inside the baby’s abdomen until the seventh month of pregnancy. It is then thought that hormones "activate" the descent of the testicles. They move slowly down from the abdomen, through the inguinal canal and into the scrotum. In most cases, the testicles should be in place shortly before, or soon after, birth.
In males, at around the ninth week of pregnancy, the male sexual chromosomes trigger the production of a hormone called testosterone. Testosterone triggers the development of the male genitals. At this stage, problems relating to absent, or atrophic, testicles can arise. For example, the penis may develop normally but, for unknown reasons, the testicles do not.
Alternatively, abnormal genital development may happen due to a rare condition, such as androgen insensitivity syndrome. This is where the body is partially insensitive to hormones, such as testosterone, which results in the baby being born with ambiguous genitalia (the genitals have male and female characteristics).
In most cases of undescended testicles, something seems to interrupt the descent of one or both testicles, and it (or they) remain inside the abdomen or the inguinal canal.
It is not known exactly why this happens, but several possibilities have been identified that increase this risk. These are outlined below.
Risk factors
The known risk factors for undescended testicles are as described below.
- Low birth weight: boys with a birth weight of less than 2.5 kg (5.5lb) are two-to-three times more likely to be born with undescended testicles than the population at large.
- 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 three times more likely to be born with the condition compared with the general population.
- Maternal diabetes: women who have type 1 and type 2 diabetes are twice as likely to give birth to a boy with undescended testicles compared with women who do not have diabetes.
- Smoking by the mother during pregnancy.
- Alcohol consumption by the mother during pregnancy (see below).
The Department of Health (DH) recommends that women avoid drinking any alcohol while pregnant. If you choose to drink alcohol while you are pregnant, do not drink more than 1-2 units of alcohol, once or twice a week. Also avoid binge drinking and getting drunk. For women, binge drinking is defined as drinking six or more units of alcohol a day.
Read more information about drinking and alcohol.
Testicular dysgenesis syndrome
Over the last 30 years, there has been a marked increase in the number of cases of undescended testicles in England and in other Western countries. Rates of three other conditions that affect the male genitals have also increased. These are:
- a low sperm count
- testicular cancer
- hypospadias: a condition where the urethra (the tube in the penis through which urine passes) is not in the correct position
Some researchers believe that all these conditions, along with undescended testicles, could be interrelated. Rather than being separate conditions, they could be different forms of a single underlying syndrome that is known as "testicular dysgenesis syndrome (TDS)".
If TDS exists, it may be caused by exposure to chemicals during pregnancy that disrupt the normal balance of hormones. This may interfere with the normal development of the male genitals.
Chemicals that are known to disrupt hormonal balance are called endocrine disruptors.
Endocrine disruptors
Examples of endocrine disruptors include:
- some types of pesticide
- polychlorinated biphenyls (PCBs): a chemical compound that is used as a coolant
- dibutyl phthalate: a chemical that is used to manufacture cosmetics, such as nail polish
In most countries, including the UK, many endocrine disruptors such as PCBs have been withdrawn due to a link with health problems. But there are still concerns that people may still be exposed to endocrine disruptors through contamination of the food chain.
The World Health Organization (WHO) has done extensive research to find out whether exposure to endocrine disruptors is causing the rise in conditions that affect male fertility.
Researchers have concluded that direct exposure to high levels of endocrine disruptors can negatively affect human health and male fertility.
But there is not yet enough evidence to prove a definite link between health problems and indirect exposure to low levels of endocrine disruptors. Indirect exposure is the type of exposure that occurs if the food chain is contaminated.
