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A pregnant woman close to giving birth.MICHAELA REHLE/Reuters

More than one in five women who have postpartum depression also suffer from bipolar disorder, and many also experience anxiety disorders and have thoughts of harming themselves, according to a new study described as the largest scale screening for depression among new mothers in the United States.

The study, published online by The Journal of the American Medical Association's JAMA Psychiatry, screened 10,000 new mothers and conducted in-home visits with more than 800 who were found to be at risk, providing researchers with a greater understanding of the onset and complications associated with postpartum depression.

While it confirmed that depression most commonly arises between four to six weeks after the birth of a child, it found that 33 per cent of cases occurred during pregnancy and nearly 27 per cent of patients experienced depression before conception.

"Because a majority of the mothers had symptoms before delivery …, we really might need to consider depression screening early on," said psychiatrist Dorothy Sit of the University of Pittsburgh School of Medicine, one of the co-authors of the study. "We may be waiting until [it] could be too late for some of these mothers."

The study's findings largely support what was previously known about postpartum depression. The condition is believed to affect between 10 and 20 per cent of new mothers, and can last up to a year after the birth of a child. (Nearly 14 per cent of mothers involved in the study screened positive for depression.)

According to the Canadian Mental Health Association, references to postpartum depression date as far back as the 4th century BC, but since it has not always been recognized as an illness, it is often underdiagnosed.

The definitive causes for the condition have yet to be confirmed, but researchers are investigating its links to biological factors, such as the major dip in hormone levels experienced after delivery, genetics, sleep deprivation and stress on circadian rhythms, and psycho-social risks, including lack of support.

Sit noted that her study's findings suggest postpartum depression is a complicated form of depression; physicians and patients need to be mindful of addressing possible additional diagnoses, including bipolar disorder, obsessive compulsive disorder, social phobia and generalized anxiety. In some cases, these other disorders may be underlying conditions that are not identified until after the patients give birth.

Despite its prevalence, however, it can be difficult for new mothers to recognize they have depression, in part, because of the chaos involved in caring for a newborn and dealing with the transition to parenthood.

"Some mothers and family members could write it off as being part of the normal experience," Sit said, but she noted that besides extreme fatigue and an inability to focus or concentrate, common signs of postpartum depression include not being able to enjoy the things one used to enjoy, not getting pleasure out of spending time with one's family and the newborn, sleeping excessively, not eating, and feeling anxious about things that never previously caused worry.

"This is not a normal part of the postpartum," Sit emphasized. "Those symptoms truly indicate it's a major depression."

Sit noted that it is important to treat depression as early as possible, especially as the study found that nearly 20 per cent of new mothers who suffer from it have thoughts of hurting themselves.

Treatment must also be tailored to address other possible disorders, since medications prescribed for depression can make patients with bipolar disorder worse. Antidepressants, for instance, can result in mania, rapid cycling, loss of sleep, agitation and irritability among those with bipolar disorder, who instead would benefit from mood-stabilizing drugs.

Patients with anxiety may also have a better chance of improving with a combination of medication and psychotherapy.