<Tr> <Td_colspan="2"> Illustration of a child with spina bifida, the most common NTD </Td> </Tr> <Tr> <Th> Specialty </Th> <Td> Medical genetics </Td> </Tr> <P> Neural tube defects (NTDs) are a group of birth defects in which an opening in the spinal cord or brain remains from early in human development . In the 3rd week of pregnancy called gastrulation, specialized cells on the dorsal side of the embryo begin to change shape and form the neural tube . When the neural tube does not close completely, an NTD develops . </P> <P> Specific types include: spina bifida which affects the spine, anencephaly which results in little to no brain, encephalocele which affects the skull, and iniencephaly which results in severe neck problems . </P>

When do neural tube defects occur during pregnancy