This article is from the Pregnancy Screening FAQ, by Lynn Gazis-Sax (email@example.com) with numerous contributions by others.
2.d In the US, about 1,600 to 2,000 babies are born with spina bifida each
year. About 800 are born with anencephaly. (from American Baby, jul 93)
The cause(s) are unknown. Having one child with a neural tube defect does
increase the probability of having another. Yet, over 95% of babies with
neural tube defects are born to families in which there is no history of
birth defects or pregnancy problems.