©Pregnancy bliss 2008

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Pregnancy and Childbirth: The answers

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Innocent causes of raised AFP (αFP)

There are other completely innocuous causes of raised AFP. These include multiple pregnancy and wrong gestational age. If the pregnancy is more advanced than the mother thinks, levels will be (erroneously) interpreted to be higher than normal.

 

Not infrequently, the cause of the raised AFP (alpha-feto protein) cannot be identified, in spite of an exhaustive search, and the baby is born healthy.

 

What follows a suspicious blood test result (raised AFP)?

The blood test is ideally done at around 16 to 17 weeks of gestation. Once the results have been obtained, a detailed anatomy ultrasound scan will be performed. This will, first of all, establish the gestational age with fairly good accuracy. The margin of error at this stage of pregnancy is, at most, one week either way.

 

The next step will be a systematic search for any possible anatomical abnormality. The cranium (skull), spine and the rest of the body will be systematically and minutely examined. A case of spina bifida or a cephalocele is usually fairly easy to identify.

 

Missing a spina bifida on ultrasound scan

The possibility of missing a spina bifida defect on a scan is pretty remote. However, like any other technical undertaking, the detection success rate will depend on such factors as the quality of the equipment used (in this case, the ultrasound machine), the expertise and experience of the operator, and the ease with which the procedure is carried out.

 

Marked obesity on the part of the mother can make a detailed ultrasound examination a very difficult undertaking.

 

Detecting "closed" neural tube defects using ultrasound scanning

As mentioned before, a blood test for estimating AFP levels will not detect closed defects. However, ultrasound scanning is very good at detecting both open and closed defects.

 

The possibility of missing a defect on ultrasound is very low. In theory, that can occur where the defect is very small and the equipment resolution poor.

 

Risk factors for neural tube defects such as spina bifida

A family history of neural tube defect is regarded as a risk factor. In fact, if one or the other parent is affected, the risk is increased ten-fold, compared to the general population. The increase in the risk is similar if one of the siblings is so affected.

 

Even with an affected second degree relative (such as an aunt or uncle), there is some increase in the risk, though modest.

 

Other risk factors for spina bifida

Diabetics on insulin have an increased risk of about four-­fold.

Epileptic mothers taking valproic acid (Epilim®) medication at conception and in the immediate period after are also at increased risk. Valproic acid is also known as sodium valproate

 

Prevalence of neural tube defects

It actually differs from country to country and from area to area in the same country. In established multi-racial societies like the United States, it has been observed that the rate is twice as high among the White population compared to the African-American population.

 

In Britain, some parts of Scotland were noted in the late 1970s to have rates which were two or three times higher than the rates in other parts of the country.

 

As for gender, girls, for some obscure reasons, are more affected than boys.

The overall prevalence is roughly estimated to be one or two per thousand. (0.1-0.2%)