Children are at increased risk of mental illness or brain development problems if their mothers suffered inflammation while pregnant, new study results say.

Inflammation caused from infection, injury and even poor diet during pregnancy may impact parts of a baby's brain associated with working memory, an international study has found.

A longitudinal study of 84 pregnant women, published in journal Nature Neuroscience, shows high levels of inflammatory marker interleukin-6 (IL-6) - a protein found in the blood - is linked to poor working memory in their children.

The researchers say they could not establish a direct causal relationship, but the results highlighted the importance of inflammation during pregnancy on a baby's developing brain.

"Importantly, this doesn't mean that every exposure to inflammation will result in a negative impact to the child; however, these findings provide new avenues for research, and can help health care providers think about how, and when, inflammation might impact a child's long-term learning development and mental health," said study co-author Alice Graham, postdoctoral fellow in behavioural neuroscience at Oregon Health & Science University.

It's long been thought heightened maternal inflammation - a response of the immune system - causes damage to the baby's developing brain, increasing the risk of mental illness or cognitive delays later in life.

To investigate this further, researchers measured blood levels of IL-6 in mothers during early, middle and late pregnancy.

After birth, they measured each infant's brain activity using functional magnetic resonance imaging to determine the pattern of the newborn's brain network organisation.

Later, at age two, they used a task to test the children's working memory -- the ability to hold items in mind temporarily.

Using machine learning techniques, the authors found a strong link between neonatal brain organisation and the level of maternal inflammation.

They also detected a link between maternal inflammation and working memory later in childhood.

Higher levels of maternal inflammation, especially late in gestation, were predictive of poorer working memory in the toddlers, the authors said.

Professor Anthony Hannan, a research fellow at The Florey Institute of Neuroscience and Mental Health, says the study adds to the understanding of how environmental exposures can influence brain development.

"Inflammation can be affected by a range of environmental factors, including infection, stress, poor diet and sedentary behaviour," Professor Hannan said.

He says the study must now be followed up by a larger cohort study of mothers and children to consider the longer-term effects, to establish whether maternal inflammation contributes to long-term brain function and cognition in adolescents and adults.