When a mother's depression goes untreated, it can negatively affect a child's cognitive development up to the age of 16, suggests new research.

The findings, published in the journal Child Development, suggest that early identification and treatment of maternal depression is key healthy growth of her children. The researchers surveyed approximately 900 healthy children and their mothers living in Santiago, Chile, at five-year intervals from the child's infancy through age 16. They observed how affectionate and responsive mothers were to their children at each age period, as well as how much mothers provided age-appropriate learning materials.

Children were assessed on verbal cognitive abilities using standardised IQ tests during each assessment. Mothers were tested for symptoms of depression. "We found that mothers who were highly depressed didn't invest emotionally or in providing learning materials to support their child, such as toys and books, as much as mothers who were not depressed. "This, in turn, impacted the child's IQ at ages one, five, 10 and 16," said Patricia East from University of California San Diego School of Medicine in the US.

"The consistency and longevity of these results speak to the enduring effect that depression has on a mother's parenting and her child's development," she added. On a scale from one to 19, the average verbal IQ score for all children in the study at age five was 7.64. Children who had severely depressed mothers were found to have an average verbal IQ score of 7.30 compared to a score of 7.78 in children without depressed mothers.

"Although seemingly small, differences in IQ from 7.78 to 7.30 are highly meaningful in terms of children's verbal skills and vocabulary," said East. "Our study results show the long term consequences that a child can experience due to chronic maternal depression," she added.  East said study data suggested approximately 20 per cent of mothers who are severely depressed when their child turns age one remain depressed for a long time. "For health care providers, the results show that early identification, intervention and treatment of maternal depression are key," said East.