When a woman with bipolar disorder becomes pregnant, she and her physician often don't realize her medication needs adjusting to prevent the symptoms from coming back -- a higher risk during pregnancy. There also is little information and research to guide dosing for psychiatric medications during pregnancy.

Approximately 4.4 million women in the U.S. have bipolar disorder with women of childbearing age having the highest prevalence.

The new study shows the blood concentration of the commonly used drug lamotrigine decreases in pregnant women. About half of the women in the study had worsening depressive symptoms as their lamotrigine blood levels dropped. The drug levels fall because women have increased metabolism during pregnancy.

"Now physicians change the dose of the drug in response to women's symptoms worsening," said lead investigator Crystal Clark, M.D., an assistant professor of psychiatry and behavioral sciences at Northwestern University Feinberg School of Medicine and a psychiatrist at Northwestern Memorial Hospital. "We need to optimize their medication dosing so they stay well."

The study results will help physicians understand how to increase their patients' doses during pregnancy and then reduce them postpartum to avoid toxicity, Clark said. Guidelines for prescribing the drug for pregnant women with the disorder also are included.

The study was published Nov. 1 in the American Journal of Psychiatry.

Depressive episodes -- as opposed to manic -- are most likely to recur in pregnant women with bipolar disorder.

"The safety of the fetus is at risk," Clark said. "Pregnant women that are depressed are less likely to take care of themselves which often leads to poor nutrition, lack of compliance with prenatal care and isolation from family and friends. It has also been linked to premature births and babies with low birth weights among other poor birth outcomes."