There are some conditions that can cause a child to wet the bed. However, in most cases there's no obvious reason why some children wet the bed while others do not.
Most experts believe that there may be more than one underlying cause.
Bladder function
The bladder is a hollow, balloon-like organ located in the pelvis and is designed to store urine. Once the bladder is full, urine passes out of the body through a tube called the urethra, located in the centre of the penis in boys and just under the main opening of the vagina in girls.
The size of the bladder is much smaller in younger children, which means that it can store less urine. Your child’s bladder won't reach full ‘adult size’ until they're 12 years old.
A five-year-old’s bladder can hold around 180ml (about a third of a pint) of urine. An adult bladder can hold 400-600ml (just over two thirds of a pint to just over one pint) of urine.
Due to the smaller size of their bladder, children are more likely to need to pass urine during the night; particularly if their urine production is higher than it should be.
Some children who are affected by bedwetting also have what's known as overactive bladder syndrome. This is where the muscles that control the bladder go into spasm, leading to the involuntary passing of urine.
Urine production
Urine is produced by the kidneys. The kidneys remove waste products from the blood. These are mixed with water to produce urine, which is then transferred into the bladder.
The more fluid your child drinks, the more urine their kidneys produce. Therefore, if your child drinks lots of fluids during the evening, it could result in them wetting the bed during the night, particularly if they have a small bladder capacity. Drinks that contain caffeine, such as cola, can also stimulate an increase in the production of urine.
Urine production is regulated by a hormone called vasopressin. In some cases of bedwetting, it may be that the child’s body doesn't produce enough vasopressin, which means that their kidneys produce too much urine for their bladder to cope with.
Nerve signals
Once the amount of urine in the bladder reaches a certain amount, the bladder should send nerve signals to the brain.
The signals should convey the feeling of needing to go to the toilet, which would cause most people to wake up. However, some younger children are particularly deep sleepers, and their brain doesn't respond to the signals being sent from their bladder, so they don't wake up.
Alternatively, in some children the nerves attached to the bladder may not yet be fully developed, so they don't generate a strong enough signal to send to the brain.
Sometimes, a child may wake up during the night with a full bladder but not go to the toilet. This may be due to childhood fears, such as being scared of the dark.
Underlying health conditions
A number of underlying health conditions can cause bedwetting and other symptoms. These are outlined below.
- Type 1 diabetes – symptoms include producing an excessive amount of urine, feeling thirsty all the time and tiredness.
- Constipation – if a child’s bowels become blocked with hard stools (faeces), it can put pressure on the bladder and lead to bedwetting.
- Urinary tract infections (UTIs) – a UTI is an infection of the urinary tract which consists of the urethra, the bladder, the kidneys and the ureters (the tubes that connect the kidneys to the bladder).
- Abnormalities with the urinary tract – such as bladder stones
- Bedwetting can also be caused by damage to the nerves that control the bladder. This could be due to an accident or a condition such as spina bifida. This type of nerve damage is known as neurogenic bladder.
Emotional factors
In some cases, bedwetting can be a sign that your child is upset or worried. Starting a new school, being bullied or the arrival of a new baby in the family can all be very stressful for a young child.
If your child has started wetting the bed after previously being dry for a period of six months or more (secondary nocturnal enuresis), emotional factors such as stress and anxiety may be responsible.
