A snapshot of GCSE maths results appears to show that people born in August do less well than those born in September.
Exams board Edexcel discovered the pattern when looking at the results from 300,000 of its papers taken in 2003 and 2004. A similar analysis of 30,000 of its English papers revealed a similar but less pronounced trend, a spokeswoman for the board said.
She added: "We found it interesting.
"We were surprised at the trend in maths and would be interested if it did follow through."
Last summer, 9.1 per cent of pupils born in September scored the top grade in maths GCSE compared with 7.5 per cent born in August, with the trend repeated for B and C grades. The conclusions were similar for exams taken in 2003, and in both years performance declined according to how late pupils were born in the school year.
In maths, the average performance in last year's exams, where an A is worth eight points and a U is worth zero, was 4.74 for September babies and 4.47 for August babies.
The spokeswoman stressed that this amounted to less than a quarter grade.
She also emphasised that date of birth was no bar to being successful in the subject, with Albert Einstein having been born in March.
It is often thought that within the current education system those born in August are put at a disadvantage by being the youngest in their year, 11 months behind those born the previous September.