Children were assessed on verbal cognitive abilities using standardised IQ tests during each assessment.
Mother's depression can lower the children's IQ and negatively affect their cognitive development up to the age of 16, a study has found.
Researchers from University of California (UC) San Diego in the US surveyed about 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 1, 5, 10 and 16," said Patricia East, research scientist at UC San Diego.
"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," East said.
On a scale from one to 19, the average verbal IQ score for all children in the study at age 5 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 said.
"For mothers in the study, there were many stressors in their lives. Most of the mothers, while literate, had only nine years of education, were not employed outside the home and often lived with extended family in small, crowded homes - factors that likely contributed to their depression," said East.
"Many mothers suffer from depression in the first six months after childbirth, but for some, depression lingers," she added.
East said study data suggested about 20 per cent of mothers who are severely depressed when their child turns age 1 remain depressed for a long time.
Future steps include further analysing the data to see how mothers' depression affects children's own depressive symptoms through childhood and adolescence.