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Pregnancy and Childbirth: The answers

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When there is a defect on one or more of the vertebrae, part of the spinal cord will not be covered. This defect is what is known as spina bifida. It is usually on the lower part of the spine.

 

Types of spina bifida

There are two major groups. The more serious type - which is immediately obvious - is where only a thin membrane covers the spinal cord from the elements. This is known as ‘open’ spina bifida.

 

The less common and less serious "occult" spina bifida is where, even though there is a defect on the bony spine, the skin over the defect is intact. This may be missed prenatally and could conceivably go unrecognized for years.

 

Only one in ten cases (10%) of spine bifida are of the ‘occult’ hidden type.

 

Skull anomalies related to spina bifida

These are known as "cephaloceles". Cephaloceles generally mean bone defects on the skull.

Because the skull closure is supposed to take place quite early in pregnancy (before six weeks of gestation), when this fails to occur, the growing brain may protrude through the defect. In about 10% of spina bifida cases, there is also a cephalocele.

 

 

 

 

 

 

 

Detecting spina bifida and related defects

The screening tests performed early in the second trimester are crucial in this. One of the chemicals looked for in the blood test variably known as the "triple test" or the "double test" is called alpha-fetoprotein or AFP (αFP).

 

AFP is a chemical produced by the fetal liver. It eventually reaches the maternal circulation by diffusion across the placenta.

 

When there is an open neural tube defect, the amount of AFP that reaches the maternal bloodstream is dramatically increased. By checking levels of AFP in maternal blood, one can therefore suspect whether there is a significant possibility of a neural tube defect such as spina bifida.

 

With ‘occult’ spina bifida and other closed defects, the AFP (áFP) levels will be normal. This test is therefore not useful in detecting this kind of defect.

 

Other causes of elevated AFP

High levels of AFP alone are not diagnostic of a neural tube defect, only suggestive. Such a result only raises the possibility of the presence of such a defect. Other causes of high levels of AFP (áFP)  include:

Ø Abdominal wall defects

Ø Placental tumours

Ø Fetal bowel obstruction

Ø Fetal skin disorders

Ø Fetal growth restriction

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A clearly visible small skull defect confirming diagnosis of a cephalocele. This particular defect was diagnosed at one year of age