Raised AFP (αFP) appears to always be a sign of something serious.
Not at all. There are innocent causes such as 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 sixteen to seventeen 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 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 easily identified.
How big is the possibility of missing a spina bifida on the scan?
Pretty remote. It is very unusual to miss a neural tube defect. 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.
Since the chemical AFP (αFP) used to screen for neural tube defects is not elevated in "closed" defects, how sensitive is ultrasound in detecting these?
Very sensitive. The possibility of missing a defect on ultrasound is very low and even then it will occur where the defect is very small and the equipment resolution poor.
Are there any known risk factors which make a mother prone to have a baby with a neural tube defect such as spina bifida?
Yes. 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 rate. 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.
Are there any other known risk factors, apart from family history?
Yes. 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
What is the general population 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 Caucasians compared to the black community. In the UK, 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%)