So, the epidemiologic study done for 8 years at CMNR Tunis has shown that the incidence is 1.15@1000 births (between 0.52 and 1.66@1000 per year). This anomaly is more frequent in female sex (sex linked = 0.59). The antenatal echographic diagnosis is efficient in 90.5%. This anomaly is more shown at low socio-economic group. The improvement of the socio-economic level and specially the daily uptake of acid folic in periconceptionnel time decrease the frequency of this congenital malformation.