If expectant mothers catch the flu during pregnancy, their babies could be four times as likely to develop bipolar disorder later in life, BBC News reported.
Bipolar disorder, typically diagnosed in the late teens or twenties, causes intense mood swings ranging from depression to feelings of manic joy. The majority of bipolar cases have no relation to the flu.
Researchers emphasized that the overall risk of flu-related bipolar remained low, leading to only a 3 to 4 percent increased chance that the child will later develop the disorder.
"The chances are still quite small. I don't think it should raise alarms for mothers,” lead study author Alan Brown said.
Brown advised expectant mothers to get a seasonal flu vaccine to reduce their chances of catching the virus.
The study, published in JAMA Psychiatry, followed 814 expectant women that gave birth in the 1960s.
Similar studies have indicated there may also be a link between the flu and schizophrenia, BBC News reported.
Dr. Fiona Gaughran, lead consultant psychiatrist at South London and Maudsley National Health Service Foundation Trust, said, "This highly regarded group of researchers has reported similar links between schizophrenia and various maternal infections.
"If future work confirms the link reported here, policymakers may need to consider implications for flu prevention pre-pregnancy, but mothers need not be worried.”