While twin concordances for schizophrenia have been used to estimate heritability and to develop genetic models, concordances in subtypes of monozygotic (MZ) twins can also be used to investigate the influence of prenatal development in the etiology of mental illness. We used within-pair variability and mirroring of fingerprints to estimate retrospectively the placentation status of concordant and discordant MZ twins. The results indicate that concordant MZ pairs were more likely to have been monochorionic (MC) and to have shared a single placenta, whereas discordant MZ pairs appear more likely to have been dichorionic (DC) with separate placentas. Pairwise concordances for MZ twins without MC markers averaged 10.7 percent. In contrast, concordances for MZ twins with one or more MC markers averaged 60 percent. This suggests that simple MZ concordance rates may overestimate schizophrenia heritability and that prenatal development may also be important in the etiology of schizophrenia. Because MC (but not DC) twins usually share fetal blood circulation and hence are likely to share infections, these results are consistent with the hypothesis that fetal infections may be a significant etiological factor in schizophrenia.