There are two main kinds of NTD (neural tube defect), spina bifida and anencephaly.
Babies with spina bifida have a defect of the formation of the lower spine. This results in a failure of closure of the bones of the spine with exposure of the spinal cord that can result in damage to the nerves controlling the lower part of the body. This causes weakness and paralysis of the legs, and sometimes bowel and bladder problems. Subjects are also more likely to have hydrocephalus (excess fluid surrounding the brain). This can be treated surgically but may lead to mental retardation.
In roughly 1515 % of spina bifida children, the spinal opening is covered with skin or tissue. This closed spina bifida is usually less severe than open spina bifida. The distinction is important from the point of view of screening, which is normally on the basis of an elevated level of AFP (alpha-fetoprotein) in the maternal serum. Leakage of fetal AFP through the opening into the amniotic fluid and then into the mother’s bloodstream causes this rise in maternal serum concentrations. With closed spina bifida the maternal AFP is not usually much raised, so subjects are not picked up using screening on the basis of AFP alone.
Fetuses with anencephaly have a large part of the skull missing, and the brain is not properly formed. Those affected always die either before, or shortly after birth. Anencephaly is an open NTD, and like open spina bifida is associated with an elevated level of AFP in the maternal serum.