Spina bifida or 'split spine' is a fault in the development of the spine and spinal cord which leaves a gap in the spine.
The spinal cord is responsible for connecting all parts of the body to the brain.
In the first month of life, a developing baby in the womb grows a structure called the neural tube. It is this that will eventually form the spine and nervous system.
But in cases of spina bifida, the spinal column - the bone that surrounds and protects the nerves - does not fully close.
The exact causes are unknown, but several risk factors have been identified, the most significant being a lack of folic acid before and at the very start of pregnancy.
There are a number of different types of spina bifida but Xiao has myelomeningocele, the most serious variant of the condition. It affects one pregnancy in every 1,000 in Britain.