We reported previously that residents of Greater Helsinki, Finland, wh
ose mothers were exposed to the 1957 influenza epidemic during their s
econd trimester of gestation had a significantly elevated risk of deve
loping adult schizophrenia. The majority of the replication studies to
date have not determined whether the mothers actually contracted an i
nfection or the stage of gestation based on mother's last menstruation
. We read prenatal clinic records of the mothers of the Helsinki-born
schizophrenia subjects to determine timing of infection, as noted by t
he prenatal clinic obstetric nurse at a time close to the actual infec
tion. Schizophrenia subjects who were exposed in the second trimester
had a significantly higher rate of definite influenza infection (86.7%
) in that period compared to those who were exposed during the first a
nd third trimesters (20.0%). These results are interpreted with cautio
n because of the small number of cases.