TUESDAY, Dec. 14 (HealthDay News) -- Babies born to mothers who are depressed during pregnancy have higher levels of stress hormones, decreased muscle tone and other neurological and behavioral differences, a new study finds.

"The two possibilities are that [the infants] are either more sensitive to stress and respond more vigorously to it, or that they are less able to shut down their stress response," lead investigator Dr. Delia M. Vazquez, a professor of psychiatry and pediatrics at the University of Michigan School of Medicine, said in a school news release.

She and her colleagues examined the association between depression in pregnant women and the development of infants' neuroendocrine system, which controls the body's stress response, as well as mood and emotions.

The study included 154 pregnant women, over the age of 20, whose depressive symptoms were assessed at 28, 32 and 37 weeks of pregnancy and again when they gave birth. Umbilical cord blood samples were taken at birth to measure stress hormone levels. At two weeks, the infants underwent neurobehavioral tests to assess their motor skills and responses to stimuli and stress.

The findings appear online and in an upcoming print issue of the journal Infant Behavior and Development.

"It's difficult to say to what extent these differences (in babies born to mothers with depression) are good or bad, or what impact they might have over a longer period of time," lead author Dr. Sheila Marcus, clinical director of U-M's Child and Adolescent Psychiatry Section, said in the news release.

"We're just beginning to look at these differences as part of a whole collection of data points that could be risk markers," she added. "These in turn would identify women who need attention during pregnancy or mother/infant pairs who might benefit from postpartum programs known to support healthy infant development through mom/baby relationships."

Up to one in five women experiences depression during pregnancy, and post-partum depression is also a common complication, according to the background material in the study. The researchers urged that pregnant women showing symptoms of depression contact a therapist.

More information

The U.S. National Women's Health Information Center has more about depression during and after pregnancy.