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"We don't fully understand this," said study co-author Dr. Alan Brown. "The best guess is it's an inflammatory response. It could also be a result of fever," he noted.
"Mothers should stay away from people who have the flu," said Brown, a professor of clinical psychiatry and clinical epidemiology at the Columbia University College of Physicians and Surgeons in New York City.
However, he added, regarding the new findings, "women should not be greatly concerned, because a fourfold increase is pretty high from an epidemiological standpoint, but still the vast majority of the offspring did not get bipolar disorder."
Brown explained that "the risk of bipolar disorder in the population is about 1 percent, so if it's increased fourfold that would make it a 4 percent risk." Moreover, the researchers only looked at one risk factor for bipolar disorder, not all risk factors, which could skew these results, he noted.
The report was published in the May 8 online edition of JAMA Psychiatry.
Bipolar disorder, also called manic-depressive illness, causes unusual shifts in mood, energy, activity levels and the ability to carry out routine tasks. Bipolar disorder can be treated, and people with this illness can lead full and productive lives, according to the U.S. National Institute of Mental Health.
The condition often develops in the late teens or early adult years. Some people have their first symptoms during childhood, while others may develop symptoms as adults, the agency noted.
For the study, researchers at Columbia University and Kaiser Permanente identified cases of bipolar disorder by database linkages of a Northern California health plan and a county health care system, along with data from a mailed survey.
Participants were mothers who gave birth between 1959 and 1966 and their offspring. Researchers found 92 cases of bipolar disorder and compared them with 722 people matched in terms of occurrence of maternal influenza during pregnancy.
While the new study found an association of pregnant women getting the flu and a higher risk of bipolar disorder in their offspring, it didn't establish a cause-and-effect relationship.
"There is no understanding of the causal factors of this," said Dr. Alan Manevitz, a clinical psychiatrist at Lenox Hill Hospital in New York City. He was not involved with the study.
"Pregnancy itself puts extra stress on women in general," he pointed out. "Pregnancy also affects the immune system and increases the risk of getting the flu."
Flu during pregnancy increases the risk of miscarriage, premature birth and low birth weight infants, Manevitz said.
Pregnant women should get a flu shot, both Manevitz and Brown suggested.
Other studies have shown a similar association between flu during pregnancy and the child's risk for autism and schizophrenia -- now there is this association with bipolar disorder, Manevitz said. "This doesn't give us any causal connection," he emphasized.
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