Table 3.

Strengths and weaknesses of individual studies

Study	Strengths	Weaknesses
NCPP birth cohort
Buka et al. 2001a	Studied several exposures, included several immunoglobulins, two-stage attempt  to case identification	Small sample and heterogeneous case group; for some cases used  maternal IgG levels without having documented seropositivity
Buka et al. 2001b	First to investigate inflammatory cytokines in these studies, included several  cytokines	Small sample and heterogeneous case group
Buka et al. 2008	Large sample allowing study of schizophrenic and affective psychosis separately	Broad definition of case ; in some cases diagnosis were made by  chart review ; overall high seropositivity in sample due to high  seropositivity among African-Americans
Xiao et al. 2009	Large sample, studied three serotypes of Toxoplasma gondii	Broad definition of case ; in some cases diagnoses were made by  chart review
Ellman et al. 2009	Examined childhood IQ and also adult schizophrenia with regard to foetal  exposure to influenza	Small sample, broad definition of cases
PDS birth cohort
Brown et al. 2000	Analysed entire cohort, large sample	Broad exposure of respiratory infections, under-reporting of  exposure and misclassification of timing of exposure is possible
Brown et al. 2004a	First serological study of prenatal influenza, good estimate of timing of exposure	Exposure based on proxy measure of seroconversion but validated  in a comparison sample with seroconversion data
Brown et al. 2004b	Larger sample size compared to a similar previous study, included several  cytokines	Median values of one cytokine (TNF-α) was lower than in most  studies of this cytokine
Brown et al. 2005	Two-stage exposure assessment, use of more sensitive tests than previous studies	Small sample size
Brown et al. 2006	Outcome more focused than previous studies, analysis restricted to seropositive  mothers	Small sample size
Babulas et al. 2006	Analysed entire cohort, large sample	Broad exposure definition, small number of exposed cases (n=5)
Danish cohorts
Mortensen et al. 2007	Large sample size	Blood samples were missing for 28% of eligible cases, short follow-  up
Sorensen et al. 2009	Analysed entire cohort, large sample, examined both narrow and broad  schizophrenia	Broad categories of exposure, viral and bacterial infections,  misclassification possible
Mortensen et al. 2010	Large sample size, detail family psychiatric history taken into account	Short follow-up
Nielsen et al. 2011	Large sample, included paternal infection and maternal infection before and after  pregnancy, family history of schizophrenia taken into account	Only able to use hospital admission for infection as exposure ;  limited power for analysis of infection during pregnancy
Finnish cohort
Clarke et al. 2009	Large sample size, used sibling comparison group, precise timing of exposure  using hospital admission data	Small number of cases meant low statistical power to examine each  trimester of pregnancy

NCPP, National Collaborative Perinatal Project ; PDS, Prenatal Determinants of Schizophrenia ; IgG, immunoglobulin G; TNF-α, tumour necrosis factor-α.