A hormone deficiency in mothers increases the chances of their children being bad at maths, research has shown.
Children of women with low levels of a thyroid hormone are 60 per cent more likely to have a poor head for sums when they reach school age than those whose mothers have normal levels, scientists found.
Previous studies had already shown that pregnant women lacking the hormone, thyroxine, are at risk of giving birth to children whose mental development is impaired in infancy.
The new findings are the first to indicate how this might affect a child's performance at school.
Lead author Dr Martijn Finken, from the VU University Medical Centre in the Netherlands, said: "Whether these problems persist into adulthood remains to be seen. We will continue to follow these children to answer this next big question."
Dr Finken's team measured thyroxine levels in the mothers of 1,196 healthy children when they were 12 weeks pregnant.
After birth, the children's progress was followed until age five, when test scores for language and arithmetic were recorded.
The study found that five-year-olds whose mothers had the lowest levels of thyroxine at the end of their first three months of pregnancy were almost twice as likely to score "subnormal" marks in the maths test.
When influences of family background and health factors were stripped out of the results, a 60 per cent difference in the test scores remained.
A "subnormal" test score was defined as coming in the bottom half of the class.
In the future, hormone tests could conceivably be used to identify children likely to need extra help in maths at school, said the researchers, whose findings appear in the European Journal of Endocrinology.
Dr Finken added: "It is possible that these children could benefit from hormonal supplements to boost their brain development in the womb. Such treatment has been tried in the past but as yet has failed to improve cognitive ability."