Interference with gastrulation during the third week of pregnancy as a cause of some facial abnormalities and CNS defects.

During the third week of pregnancy the human embryo undergoes a major developmental process, gastrulation, during which the two-layered embryo is converted into a three-layered embryo. At the same time, the upper epiblast layer is induced to form the neural plate. Evidence is presented which suggests that interference with this process by genetic, physical, or chemical agents can cause a range of CNS abnormalities and facial abnormalities, including those described as characteristic of the FAS.