The finding, published in The Journal of Child Psychology and Psychiatry, give researchers insight into why postpartum depression seems to be linked more often to Caesarean birth.

The researchers based their findings on MRI scans that show heightened activity in the sections of the brain thought to regulate emotions, motivation and habitual behaviors. They believe this may be because vaginal childbirth involves the release of oxytocin -- a key mediator of maternal behavior in animals -- from the posterior pituitary, uterine contractions and vagino-cervical stimulation.

The researchers also studied areas of the brain affected by delivery conditions and found ties between brain activity and measures of mood. This suggests that some of the same brain regions may help regulate postpartum mood.

"As more women opt to wait until they are older to have children, and by association be more likely to have a Caesarean-section delivery, these results are important, because they could provide better understanding of the basic neurophysiology and psychology of parent-infant attachment," lead author James Swain, of Yale University's Child Study Centre, said in a news release issued by the journal's publisher. "This work could lead to early detection of families at risk for postpartum depression and attachment problems and form a model for testing interventions."

Caesarean delivery, which occurs via incisions in the abdominal and uterine wall, is required at times to protect the health or survival of infant or mother. The procedure's use has increased in the United States dramatically, from 4.5 percent of all deliveries in 1965 to a recent high in 2006 of 29.1 percent.

More information

The U.S. Department of Health & Human Services has more about postpartum depression.