• Congenital defects are not all genetically determined at the moment of conception; many are acquired during the ensuing fetal development. The latter are usually fetal manifestations of critical stress on the mother during pregnancy. Just as the genetically determined defects have been studied in the fruit fly by breeding experiments, so the acquired defects have been studied in the gravid mouse by using hypoxia as a standard stress at different stages in gestation.
From these latter experiments a whole panorama of deviations emerges, determined as to kind and severity by the timing and degree of the stress applied to the mother.
A large class of congenital defects is therefore preventable. They need to be attacked with the same energy that is now being directed at poliomyelitis and other causes of disability.