They have found that young infants find the flat tones of depressed mothers less stimulating than the excited high-pitched ones associated with baby talk.

They say this could explain why the children of depressed mothers do not perform as well in child development tests as most other children.

At least one in 10 women in the UK are thought to suffer from post-natal depression.

The researchers from the University of Colorado said babies who were exposed to the recordings of depressed mothers were slower in tests.

They studied 225 babies aged about four months.

They then recorded mothers with varying degrees of clinically diagnosed depression trying to get their infants interested in a toy gorilla.

The mothers said "pet the gorilla" to the babies.

The mothers who were severely depressed pronounced the words in a flatter tone than those who were only moderately depressed.

The babies were played the recordings and then shown a videotape of a smiling mother.

Stimulation

While they listened to the recording, a pattern was put in front of them and the researchers studied how much time they spent looking at it.

They found that children who listened to severely depressed women spent much less time looking at the pattern.

Professor Peter Kaplan, who led the study, said: "We found that depressed mothers who were trying to interest their infants in a toy - a stuffed gorilla - said the word 'gorilla' in a voice with relatively flat pitch.

"This raised the possibility that infants, who are known to react more strongly to speech high in pitch modulation, would not learn well about the world around them when prompted with speech uttered by depressed caregivers."