The children of women who have their first child in their early twenties appear to do much worse at school then those born to teenage or older mothers.

Cardiff University researchers believe the stresses that accompany becoming a mother aged 20-22 undermine the ability of women to help their children gain a good education.

They found that children whose mothers first became pregnant in their late twenties or early thirties were almost three times as likely to get at least five GCSEs at grades A*-C. Even those born to women who first became mothers aged 16-19 did twice as well as those whose mothers did so in their early twenties.

Prof Dale Hay, developmental psychologist at Cardiff University, believes that this finding can be explained by teenage mothers having more support either from social services or because they lived at home.

Speaking at the annual conference of the British Psychological Society in Cardiff yesterday, Prof Hay said: "It is not about how old they were when they had this child, but how old they were when they started having children.

"There has been a lot of controversy about the obstetric dangers of delaying parenthood, however the older group we looked at had the best educational outcomes."

Researchers divided 165 children born to women in Lambeth and Thamesmead in 1986 into three groups based on the age at which their mother had her first child and examined their GCSE results achieved in 2002-03 and 2003-04.

The children were not necessarily first-borns. Among the children whose mothers were aged 16 to 19 at the time of their first child, 43 per cent got at least five GCSEs at grades A*-C.

Only 22 per cent of those born to women who were between 20 and 22 when they first became mothers attained the same grades.

In those born to mothers who waited until 23 or older when they had their first child, the proportion was 61 per cent.

The national average of children achieving at least five GCSEs at grades A*-C was a little above 50 per cent during those years.

Data collected on the numeracy and literacy skills of the children in their last year of primary school found those whose mothers had their first child in their early twenties were already falling behind by then.