During the first month of pregnancy, the two sides of the spine close to cover the spinal cord and it’s covering tissues. Incomplete closure of the spine results in a birth defect called as spina bifida and this results in the spinal cord and it’s covering tissues sticking out as a sac on the child’s back.
Spina bifida can be of different types like spina bifida occulta (in which the spinal cord and meninges are in their place but the bones of the spine do not close), meningocele (the membranes responsible for covering and protecting the brain and spinal cord i.e. the meninges push through the spinal defect into a sac called a meningocele).
Myelomeningocele is the most common type of spina bifida and so the terms are often used interchangeably.