Chicago: First-time mothers are at increased risk for mental disorders while fathers do not share that risk, Danish researchers said on Tuesday.

In a 32-year study that included more than 2.3 million Danes, roughly one out of 1,000 first-time mothers were admitted to a psychiatric hospital with a mental illness such as depression, schizophrenia or bipolar disorder within the following year - most within the first three months.

Mothers were four times more likely than women without children to be hospitalised with a mental illness. During the first 10 to 19 days, the risk was seven times higher compared to women with children who had reached 1 year of age.

Among fathers, only 0.37 of 1,000 births resulted in a mental disorder, which was comparable to men without children or men with an older infant.

"This may indicate that the causes of postpartum mental disorders are more strongly linked to an altered physiological process related to pregnancy and childbirth than psychosocial aspects of motherhood," said study author Trine Munk-Olsen of the University of Aarhus, Denmark.

The drastic changes in a woman's hormones before and after the birth may be the main reason behind the mental problems, the report suggested, with the demands of breastfeeding and sleep deprivation contributing to the stress.

Postpartum depression has been estimated to affect as many as 15 percent of mothers and can be debilitating. An editorial accompanying the study published in this week's issue of the Journal of the American Medical Association urged universal screening for postpartum mood swings, especially in the first three months and particularly when the birth is the mother's first.

"Effective treatments for these episodes exist," Katherine Wisner of the University of Pittsburgh Medical Center wrote. "Maternal depression exacts a heavy toll on women's functioning and the health and well-being of their children."