FRIDAY, Dec. 9 (HealthDay News) -- New mothers who experience a psychiatric disorder within 30 days after giving birth have an increased risk of developing bipolar disorder, according to a new study.

Researchers examined data from more than 120,000 Danish women born between 1950 and 1991 who had received first-time psychiatric care either as an outpatient or an inpatient for any type of psychiatric disorder other than bipolar disorder. Of those women, 2,870 had first-time psychiatric contact within a year of giving birth to their first child.

During follow-up, more than 3,000 of the 120,000 women were diagnosed with bipolar disorder. Of those, 132 had first-time psychiatric contact within a year after giving birth.

Fifteen years later, bipolar disorder had been diagnosed in nearly 14 percent of women with initial contact within 30 days after giving birth compared with less than 5 percent of women who had initial contact one month to one year after giving birth, and 4 percent of those with initial contact one year or more after giving birth.

Twenty-two years later, bipolar disorder had been diagnosed in 19 percent of women who had initial contact within a month of giving birth, compared with 6.5 percent of those who had initial contact within a month to a year after childbirth, and 5.4 percent of those with initial contact one year or more after giving birth.

The study appears online in the journal Archives of General Psychiatry.

"Childbirth has an important influence on the onset and course of bipolar affective disorder, and studies have shown that episodes of postpartum psychosis are often best considered as presentations of bipolar affective disorder occurring at a time of dramatic psychological and physiological change," the researchers wrote. "It is also clear, however, that a high number of women with the new onset of a psychiatric disorder in the immediate postpartum period do not receive a diagnosis of bipolar disorder."

"The present study confirms the well-established link between childbirth and bipolar affective disorder and specifically adds to this field of research by demonstrating that initial psychiatric contact within the first 30 days postpartum significantly predicted conversion to bipolar affective disorder during the follow-up period," the study authors concluded. "Results indicate that the presentation of mental illness in the early postpartum period is a marker of possible underlying bipolarity."

While the study suggests an association between psychiatric episodes soon after childbirth and bipolar disorder, it does not show cause and effect.

More information

The U.S. National Institute of Mental Health has more about bipolar disorder.