The tell-tale sign of postnatal depression that has long been ignored.

A new study has revealed there’s one symptom of postnatal depression which has been overlooked that can actually go a long way in affecting recovery.

According to a study by the University of British Columbia, while most women are screened for signs of anxiety and depression, they’re not actually being checked for signs of anger.

In fact, anger doesn’t even appear on the commonly used Edinburgh Postnatal Depression Scale screening tool.

The study was conducted by nursing PhD student Christine Ou, who discovered anger actually plays quite a large role in postnatal depression, and can even hinder a woman’s recovery if left untreated.

"We know that mothers can be depressed and anxious in the postpartum period, but researchers haven’t really paid attention to anger," Ou told the University of British Columbia.

"There’s some evidence that indicates that being both angry and depressed worsens the intensity and length of depression. That can have many negative effects on the mother, child and family, and on the relationship between parents."

Ou discovered many women feel anger as a direct result of motherhood not living up to their expectations.

She said many mothers also felt angry about having their parenting judged by others, and not having the support they had hoped for from those closest to them.

"Mothers may feel that they have not met their own expectations and that also others may judge them because, for example, they’re formula-feeding instead of breastfeeding," she told the University of British Columbia.

"Many mothers have also expressed feeling let down by others in terms of support from partners, family members, and health-care providers as well."

She also explained that many cultures make it unacceptable for women to express anger, which can often transition into depression.

According to Ou’s thesis supervisor and co-author, University of British Columbia nursing professor Wendy Hall, this discovery will play an important role in the successful treatment of postnatal depression moving forward.