Down syndrome (DS) is the commonest chromosomal disorders in the world; affecting all countries, all races, and both sexes. It was identified since premodern art and middle ages. The risk for DS births is multi-factorial and includes both genetic and environmental factors. The prevalence of DS could be affected by different factors including distribution of maternal age in the population, adequacy and completeness of ascertainment, accurateness of diagnosis, level of selective prenatal termination of affected pregnancies, as well as different unrecognized genetic and environmental factors. Incidence of DS is expected to be significantly high in the developing countries, probably due to the higher death rate from comorbidities in DS such as congenital cardiovascular defects. Improving survival of infants with DS because of better care especially of cardiovascular malformations will affect prevalence rather than the incidence of DS. According to World Health Organization; the predictable incidence of DS is between 1 in 1,000 to 1 in 1,100 live births all over the world. The difference in prevalence among populations or countries or in the same population over time will depend on the potential risk factors in common for that community.
Keywords: Africa, America, Arabs area, Asia, Chemical Toxins, Children, Chromosome aberrations, Consanguinity, Cytogenetic, Down syndrome, Environmental factors, Europe, Genetic Factors, Gonadal trisomy mosaicism, Incidence, Ionizing radiation, Maternal age, Paternal age, Prevalence, Smoking, Socioeconomic Status.