Abnormalities of any part of the human body may develop during pregnancy. They may be relatively minor (such as excess toes), not affecting the health of the individual, or they may be very serious and lead to physical disability and even death. Congenital abnormalities are due to imperfections in the genetic information which has affected the developmental process.
Cases include rudimentary or seal-like limbs, incomplete or undeveloped extremities, fusion of the lower extremities, absence of the brain, abnormally small brain and head, enlargement of brain and head, protrusion of the brain through the head, absence of eyes, abnormally small eyes, defective closure of lines of junction (harelip, cleft palate, spina bifida), partial or wholly double individuals (Siamese twins), or individuals with double heads or bodies. Many such conditions may be regarded as resulting from the action of an unfavourable environment on a genetically susceptible individual. The relative importance of the environment varies with the condition.
Data of The International Clearinghouse for Birth Defects Monitoring Systems (of the World Health Organization) monitors the incidence of specific congenital birth defects from over 25 centres in various parts of the world.
Across the world, 4-8% of all babies are born each year with defects affecting one or more organs. Of these children, 75% show some anomaly of the head or face.
The frequency of such traits reported in all countries has been estimated to be 1.5% of the live-born; higher if still-births are included. An additional 1% of affected children can be detected by the age of 5 years. Many unsuspected abnormalities can only be detected by laboratory tests and x-ray. With due reservations concerning differences in criteria, data from the WHO over recent years indicate cases of this problem in the following countries:< [Africa] Mauritius (57). [America] Bahamas (22), Barbados (16), Belize (4), Brazil (8,544), Canada (1,469), Chile (1,287), Costa Rica (309), Dominican Rep (402), El Salvador (172), Guatemala (719), Haiti (5), Martinique (7), Neth Antilles (14), Panama (157), Paraguay (130), Puerto Rico (194), St Christopher-Nevis (2), St Lucia (2), Suriname (25), USA (13,938), Virgin UK (1). [Asia] Hong Kong (269), Israel (452), Japan (4,345), Kuwait (302), Singapore (177), Sri Lanka (434), Syria (177), Thailand (660). [Pacific] Australia (937), New Zealand (226), Papua New Guinea (22). [Europe] Austria (409), Belgium (488), Bulgaria (559), Czechoslovakia (1,111), France (2,599), Germany (West) (2,458), Greece (777), Hungary (915), Iceland (19), Ireland (358), Italy (2,910), Luxembourg (12), Malta (33), Netherlands (770), Northern Ireland (135), Poland (3,948), Romania (1,956), Scotland (298), UK (England and Wales) (3,037), Yugoslavia (1,317).
The incidence of physical birth defects is falling in many industrialized countries due to the improvements in the resolution of prenatal ultrasound and other fetal diagnostic tools. Parents often choose to abort a child who will be born with disabilities.