Flu during pregnancy may raise an unborn child's risk of getting bipolar disorder in later life.
That is the suggestion of a new study of 814 expectant women.
It found that bipolar disorder was nearly four times as common in people whose mothers caught flu during pregnancy.
Bipolar is a condition affecting around one in 100 people which can last for months.
It can bring great mood swings, ranging from depression and despair to manic feelings of joy, over-activity and loss of inhibitions.
Report authors say that policymakers may have to consider pre-pregnancy flu prevention measures if future similar studies confirm the link.
Experts at the Columbia University Medical Centre associated the condition, often diagnosed during late teens and 20s, with experiences in the womb.
Researchers stress, however, that the overall risk remains small and that women should not worry.
The lead researcher, Prof Alan Brown, calculated that flu infections during pregnancy could result in a 3-4% chance of bipolar disorder in the resulting children.
There would be no history of flu, he said, in a huge majority of bipolar cases.
Dr Fiona Gaughran, lead consultant psychiatrist at South London and Maudsley NHS Foundation Trust, said mothers need not be worried.
She said: "The overall risk of offspring developing bipolar disorder is low, even if one did get flu in pregnancy."
The research, which mirrors similar results associating flu with schizophrenia, looked at people born in the early 1960s.
Its findings are published in JAMA Psychiatry.
Copyright Press Association 2013