Spina bifida is a type of neural tube defect. Neural tube defects, including spina bifida (open spine) and anencephaly (open skull), are seen in one out of 1,000 pregnancies.
During pregnancy, the human brain and spine begin as a flat plate of cells, which rolls into a tube, called the neural tube. If all or part of the neural tube fails to close, leaving an opening, this is known as an open neural tube defect (or ONTD). This opening may be left exposed (80 percent of the time), or covered with bone or skin (20 percent of the time).
Anencephaly and spina bifida are the most common types of ONTD, while encephalocele (in which there is a protrusion of the brain or its coverings through the skull) is much rarer. Anencephaly occurs when the neural tube fails to close at the base of the skull, while spina bifida occurs when the neural tube fails to close somewhere along the spine.
In over 95 percent of cases, an ONTD occurs without a prior family history of these defects. ONTDs result from a combination of genes inherited from both parents, coupled with environmental factors. For this reason, ONTDs are considered multifactorial traits, meaning "many factors," both genetic and environmental, contribute to their occurrence.
Some of the environmental factors that may contribute to ONTDs include uncontrolled diabetes in the mother, and certain prescription medications. According to the Centers for Disease Control and Prevention (CDC), the occurrence rate of ONTDs can vary from state to state and from country to country.
ONTDs are seen five times more often in females than males. Once a child with an ONTD has been born in the family, the chance for an ONTD to occur again is increased to 3 to 5 percent. It is important to understand that the type of neural tube defect can differ the second time. For example, one baby could be born with anencephaly, while a second baby could have spina bifida (not anencephaly).