Our findings in the Helsinki Influenza Study and the Danish Forty Year Study lead us to conclude that a 2nd-trimester maternal influenza infection may increase risk for adult schizophrenia or adult major affective disorder. More recently we have also reported an increase of unipolar depression among offspring who were exposed prenatally to a severe earthquake (7.8 on the Richter scale) in Tangshan, China. Among the earthquake-exposed males (but not the females), we observed a significantly greater depression response for those individuals exposed during the 2nd trimester of gestation. These findings suggest that maternal influenza infection and severe maternal stress may operate (in different ways) as teratogens, disrupting the development of the fetal brain and increasing risk for developing schizophrenia or depression in adulthood.