Women who give birth to boys increase their risk of post-natal depression by 79% over those having a girl, say scientists.

But a difficult birth triples their risk no matter what the sex of the child, researchers found.

A study by the University of Kent, which is based in Canterbury and Chatham Maritime, claims male infants cause more inflammation during pregnancy than female ones - an immune response that has been linked to mental illnesses.

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The baby blues or PND (post-natal depression) affects one in every 10 women within a year of giving birth.

Identifying those most at risk could lead to vulnerable women getting the support and treatment they need at the earliest possible opportunity.

Co-author Dr Sarah Johns, of the University of Kent, said: "PND is a condition that is avoidable.

"It has been shown giving women at risk extra help and support can make it less likely to develop.

"This might also be the reason why the study found women already experiencing depression or anxiety were at a lesser risk than their peers - because they already had support systems in place.

"The finding that having a baby boy or a difficult birth increases a woman's risk gives health practitioners two new and easy ways to identify women who would particularly benefit from additional support in the first few weeks and months."

Previous studies have discovered a link between inflammation and the development of depression.

But while carrying a male baby has been shown to increase the immune response it was not known to fuel PND - until now.

"The finding that having a baby boy or a difficult birth increases a woman's risk gives health practitioners two new and easy ways to identify women who would particularly benefit from additional support in the first few weeks and months..." - Dr Sarah Johns, University of Kent

The finding published in Social Science and Medicine was based on a study of the complete reproductive histories of 296 women.

Those who gave birth to males were 71% to 79% more likely to develop PND.

The analysis also showed mothers whose births had complications were nearly three times more prone. This could also help screen for the condition.

Dr Johns and colleagues said male infants and birth complications should be seen as PND risk factors and used by health professionals to identify parents most at risk.

PND is a type of depression that many parents experience after a baby is born. It mainly affects mothers but can also affect fathers or partners.

To find out why boys are more likely to cause PND than girls the researchers looked at the link between participants' inflammatory immune response and the development of depressive symptoms.

This is because known risk factors for depressive symptoms are associated with activation of inflammatory pathways.

They found both the development of male foetuses and the experience of birth complications have documented associations with increased inflammation.

"It’s important to seek help as soon as possible if you think you might be depressed, as your symptoms could last months or get worse..." - Dr Sarah Johns, University of Kent

But Dr Johns said more research needs to be done in this area to establish other causes of why baby boys may increase PND rates.

The NHS says symptoms of PND include a persistent feeling of sadness and low mood, lack of enjoyment and loss of interest in the wider world.

It causes a lack of energy too along with feeling tired all the time, trouble sleeping at night and feeling sleepy during the day, difficulty bonding with your baby, and withdrawing from contact with other people.

It may also trigger frightening thoughts – and even lead to mothers considering hurting their baby.

Dr Johns said: "A growing body of literature links both depressive symptoms generally, and those specifically in the postnatal period, with an inflammatory immune response."

Previuous research in other mammals has likened depression to sickness behaviour - suggesting it's a protective mechanism when an individual is experiencing a disease threat.

Dr Johns said: "Many known risk factors for depressive symptoms are associated with activation of inflammatory pathways - opening up the potential for identifying novel risk factors based on their inflammation causing effects.

"Both the gestation of male foetuses and the experience of birth complications have documented associations with increased inflammation, yet their relationships with PND are currently unclear.

"These results highlight two novel PND risk factors, male infants and birth complications, which can be easily assessed by health professionals. "

Added Dr Johns: "It’s important to seek help as soon as possible if you think you might be depressed, as your symptoms could last months or get worse."