So it can be linked to a disease, not just random effects:
Apparently getting sick while you’re pregnant sucks in even more ways than you realized. Children are more likely to develop bipolar disorder later in life if their mothers caught the flu while pregnant, a new study published yesterday in JAMA Psychiatry suggests. The effect is fairly small, amounting to just a 3% to 4% bump in risk, and most people with bipolar disorder had no history of in utero flu. But it’s an interesting finding because it echoes earlier research linking maternal flu with schizophrenia, the BBC observes.