EPIDEMIOLOGY

According to estimates by the Centers for Disease Control (CDC), spina bifida and anencephaly, the two most common neural tube defects (NTDs), affect approximately 3,000 pregnancies yearly in the United States.

It is notable that affected pregnancies in both Hispanic and non-Hispanic whites have declined sig­nificantly since the mandatory fortification of grain products in the United States with folate (see the dis­cussion in the section “Genetic Influences”). Several trials have shown that folic acid fortification can alter a woman's risk of an NTD-affected birth from 50% to 70%. Although the impact of folic acid on NTDs does not appear to be influenced by race or ethnicity, the disparity between Hispanics and other races and ethnicities remains, and the causes are unknown at this time. It is hypothesized that differences in eat­ing habits, supplement-taking practices, and in gen­eral an awareness of how nutrition affects pregnancy outcomes has a major impact. Indeed, differences in social structure may play a role. Studies have demon­strated also that other risk factors such as maternal obesity, nutrient intake, and supplement use are dif­ferent, depending on racial/ethnic grouping. Genetic factors, some of which are discussed in a later sec­tion, have a major direct impact on NTD incidence and serve to alter susceptibility to numerous environmen­tal influences. It is this interaction that requires con­tinued study in order to discern how racial and ethnic factors change over time and affect NTDs.

The current American Academy of Pediatrics (AAP) guidelines for folic acid supplements are:

■ All women of childbearing age: 400 micrograms or 0.4 milligrams/day

■ Women with a previous NTD pregnancy: 4000 micrograms or 4 milligrams/day one month prior to conception and through the first trimester

■ High-risk pregnancies (such as a mother who is tak­ing valproic acid or has maternal diabetes): 4 mil- ligrams/day (1)

Understanding the multifactorial etiology of this disorder is complex, but the underlying genetic risks are well established. Although recurrence risk varies around the world, most children are born to families without a prior affected child (0.1%-0.2%). The risk for recurrence in a family with one child with NTD is 2% to 5% and increases to the 10% to 15% range if two siblings are affected. If one parent has spina bifida, the risk is 4% of having a child with a similar disorder. For a recent review, see Deak et al. and the NTD Collaborative Groups work (2).