Some studies have suggested that being exposed to influenza in the womb increases your risk of schizophrenia. For example, Finnish people whose mothers were in the second trimester during the 1957 flu epidemic had a higher rate of schizophrenia. But the data are far from conclusive; many studies have found no relationship at all. In a new study, researchers looked more closely at schizophrenia patients and their mothers.

What the researchers wanted to know: Does prenatal exposure to influenza increase the risk of schizophrenia?

What they did: The researchers took advantage of the Child Health and Development Study, which ran from 1959 to 1966 and enrolled nearly all of the pregnant women in a California county who were in the Kaiser Foundation Health Plan—and their children. For this study, they picked people out of the 12,094 children who belonged to the same health plan between 1981 and 1997. Seventy-one had schizophrenia and related disorders, and for 64 of them, the researchers could get blood serum samples taken from their mothers during pregnancy. The offspring with schizophrenia were compared with controls who had no major psychiatric disorders.

What they found: People whose mothers had been exposed to influenza in the first trimester of pregnancy appeared to have a much higher risk of schizophrenia and related disorders. The difference isn't statistically significant, but it's big enough to pay attention to anyway.

What the study means to you: Yet another study falls short on conclusively linking schizophrenia and influenza, but that doesn't prove they're not linked—they may well be. It's just a reminder that it's really, really hard to prove this kind of connection.

Caveats: The mothers' blood samples had been in the freezer for over 30 years, but the researchers said they were probably OK.

Find out more: The CDC recommends flu shots for pregnant women.

Read the article: Brown, A.S. et al. "Serologic Evidence of Prenatal Influenza in the Etiology of Schizophrenia." Archives of General Psychiatry. August 2004, Vol. 61, pp. 774–780.

Abstract online: http://archpsyc.ama-assn.org