The study of birth defects has assumed an importance even greater now than in the past because mortality rates attributed to congenital anomalies have declined far less than those for other causes of death, such as infectious and nutritional diseases. It is estimated that as many as 50% of all pregnancies terminate as miscarriages. In the majority of cases this is the result of faulty development. Major congenital malformations are found in at least 2% of all liveborn infants, and 22% of all stillbirths and infant deaths are associated with severe congenital anomalies. Teratological studies of an experimental nature are neither ethical nor justifiable in humans.
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