Dear WW: atresia is not a rare condition. I still remember being told in embryology that every newborn baby should have little finger of obstetrician very cautiously inserted into its rectum to detect imperforate anus. An excess of caution. Little girls may have imperforate hymen,but that is seldom troublesome and easily corrected. Embryology is fascinating. There are so many changes made so rapidly that it is amazing that there are not far more congenital defects.