Middle-class parents may be timing the birth of their babies to improve their children's chances of doing well at school, according to Government-backed research.
A major study found children born at the beginning of the academic year in September perform far better in exams than pupils who are "young for their year", those born in July or August.
The "conception decisions" of middle-class parents who aim to give birth in the autumn may be increasing the divide between rich and poor, the researchers said.
Poorer families, whose children are entitled to free school meals, appear not to be making such deliberate calculations, according to academics who conducted the research for the Institute for Fiscal Studies said.
The findings suggest that family planning decisions may be partly behind one of the biggest problems facing education - the continuing poor performance of children from working-class homes.
Researcher Professor Lorraine Dearden, from the University of London's Institute of Education, said proportionately more children on free school meals were born in the summer than in the autumn.
She said: "It could be that middle-class parents are choosing when to have their children or it could be that if a middle-class parent has an August-born child, they don't put them in the state sector, whereas if they have a September-born child, they do."
Co-author Professor Costas Meghir, from University College London, added: "The reaction of the middle-classes to this kind of knowledge may well be exacerbating inequalities."
The research report, When You Are Born Matters, raised the possibility that "wealthier parents deliberately decide to have children in the autumn rather than the summer".
The report considered the idea that "parents from certain backgrounds try to ensure that their child will always be one of the oldest in the school year through conception decisions".
Another suggestion made was that middle-class children "who are amongst the youngest in their year are more likely to be put into private schools".
The report concluded: "Initial analysis of our sample suggests that there is some evidence that children who are eligible for free school meals (a proxy for low income) are more likely to be born in August than children who are not eligible for free school meals."
The study did not examine the position in the private sector, but included data for every child in state schools in England.
Children who are born in August fall far behind their classmates, the report said.
The later a child is born in the academic year, the worse their grades are likely to be.
Even aged 16, after 11 years of schooling, the impact of a pupil's month of birth can still be seen in GCSE results, with serious implications for their chances at A-level and university.
The Government-funded research by the Institute for Fiscal Studies called for an "urgent" action to address the issue, including adjusting pupils' test scores to take account of their age.
Another option would be to allow more flexibility in the timing of when children have to start school.
Ministers said they were considering the "very interesting" report.Reuse content