fetuses are undoubtedly aborted spontaneously, early in pregnancy. Nevertheless, worldwide, the prevalence of neural tube defects occurring at birth appears to be falling. This can be partly attributed to screening programmes and possibly to improvements in nutrition, but the rate also seems to be declining for unexplained reasons. In the UK, the prevalence throughout the country is falling from an overall level of approximately 4 per 1,000 live births in the 1970s to in the region of 0.3 per 1,000 live births now. There remains a significant geographical variation in the prevalence of cases in this country, with the highest rates occurring in the west of the country, namely in western Scotland, Wales and Northern Ireland, where the incidence may be more than twice as high as elsewhere, but the incidence in these areas is also falling. Elsewhere in the world, the prevalence rates are generally lower, ranging down to about 1 in 10,000 in sub-Saharan Africa.
There is a significant genetic component to the development of neural tube defects, since, if either parent has had an affected child or if either parent is affected by the condition, there is an approximately 10% risk of further offspring having a neural tube defect. If two affected pregnancies occur, the risk to a further pregnancy is increased about 20-fold. Secondary prevention of further affected pregnancies requires screening techniques, including alphafetoprotein sampling and ultrasound, with selective termination of affected pregnancies. Primary prevention, however, requires the prevention of neural tube defects occurring in the embryo in the first place, even in pregnancies not at higher risk of such an occurrence. Since the 1960s, it has been recognized that women with an affected pregnancy had significantly lower red cell folate levels than those with unaffected pregnancies. The Medical Research Council Vitamin Study of 1991 , in which women who had had a previous pregnancy affected with a neural tube defect were randomized to receive folic acid (4 mg daily) or placebo, with or without other multivitamin supplements, demonstrated that the rate of affected subsequent pregnancy was significantly reduced in the folic acid group (relative risk 0.29). It is now, therefore, recommended that in order to prevent a recurrence of a neural tube defect in subsequent pregnancies, 5 mg daily of folic acid should be supplemented to the diet prior to conception. In order to prevent a first occurrence of a neural tube defect, all women should be advised to take 400 mg of folic acid daily prior to conception, as well as increasing their dietary intake of foods rich in folic acid, continued until the 12th week of pregnancy (food high in folate includes green vegetables, yeast, beef extract and breakfast cereals fortified with folic acid) .
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