Anencephaly is a lethal malformation that originates early in development and is related to the non-closure of the anterior neuropore. As a result the main part of the brain does not develop properly but lies as a disorganized mass on the exterior of the head. Many anencephalic infants also have malformations on other parts of the body. An anecephalic infant is stillborn or expires soon after birth.
The rate of anencephaly in various countries (minimum 10 year average) ranges between 0.8 and 18.4 per 10,000 infants, most averaging between 4 to 5 per 10,000, and has tended to decline in the past decade. Mexico, Northern Ireland and Japan have rates at the upper end of the range (18.4, 11.5 and 9.2 per 10,000, respectively), but are stable or declining except for Mexico, which has shown a remarkably increasing rate in the 1980s (though not South America as a whole). A slightly higher rate in Norway than other Nordic countries can be an effect of a much lower stillbirth limit in the former country than the others (16 vs 28 weeks). Both the declining time trends and the differences in rates around the world may largely be the result of prenatal diagnosis followed by elective abortion.