Flu during pregnancy may increase the risk of the unborn child developing bipolar disorder later in life, research suggests.A study of 814 expectant women, published in JAMA Psychiatry, showed that infection made bipolar four times more likely.The overall risk remained low, but it echoes similar findings linking flu and schizophrenia.Experts said the risks were small and women should not worry.Bipolar leads to intense mood swings, which can last months, ranging from depression and despair to manic feelings of joy, overactivity and loss of inhibitions.Researchers at the Columbia University Medical Center identified a link between the condition, often diagnosed during late teens and twenties, and experiences in the womb.In their study looking at people born in the early 1960s, bipolar disorder was nearly four times as common in people whose mothers caught flu during pregnancy.The condition affects about one in 100 people. The lead researcher, Prof Alan Brown, estimated that influenza infection during pregnancy could lead to a 3-4% chance of bipolar disorder in the resulting children.However, in the vast majority of cases of bipolar disorder there would no history of flu.Seasonal vaccinationSo in the list of things pregnant women have to worry about, how high should it rank?"I wouldn't say high," Prof Alan Brown told the BBC."The chances are still quite small. I don't think it should raise alarms for mothers."He said seasonal flu vaccination, which is advised for pregnant women in many countries, would reduce the chances of catching flu.Similar studies have shown a link between flu and schizophreniaHow flu could affect the foetal brain has not been completely explained.Influenza is not thought to directly affect the foetus, but the mother's immune response to the virus could affect development.Dr Fiona Gaughran, lead consultant psychiatrist at South London and Maudsley NHS 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."The overall risk of offspring developing bipolar disorder is low, even if one did get flu in pregnancy."