To test the hypothesis that in utero exposure to maternal influenza is associated with developing bipolar disorder as an adult, the researchers first had to identify all members of the Child Health and Development Study who developed the disorder after childhood. To do this, they reviewed Kaiser's medical database and records from Alameda County Behavioral Health Care Services, and sent questionnaires to all study participants and their mothers. After all possible cases were identified, a structured clinical interview was conducted with each person to independently confirm bipolar disorder. There were 92 cases of the disorder.

The finding: 8.7 percent of mothers of the adults with bipolar disorder had had the flu at some point while they were in the womb compared to 2.6 percent of the mothers of adults without bipolar disorder. This strong association persisted after controlling for factors such as maternal mental health, age, race, educational level, and gestational age of birth. Maternal exposure to influenza was more common among people with bipolar disorder across all three trimesters of pregnancy and the period just before and just after conception.

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