Spina bifida is a condition in which the spine fails to close during the first month of pregnancy. In most cases of open spina bifida, a fluid-filled sac protrudes from the injury site. It contains the deformed nerves and the membrane that surrounds them, called the meninges. This sac is called a myelomeningocele. Less commonly, the spine splits backward and is not covered by the meninges; this is called a myelomeningocele.