"Exposure to pesticides and nitrates can alter the hormonal milieu of the pregnant mother and the developing foetal brain."
Dr Winchester, of Indiana University School of Medicine, looked at how the exam performance of more than 1.5million children aged between eight and fifteen years old varied with month of birth. Levels of pesticide and fertiliser use throughout the year were also monitored.
Analysis showed that those conceived in the summer tended to do significantly worse in maths and English.
Presenting findings to the Pediatric Academic Societies' annual conference in the USA, the researcher said that while he hadn't actually proved that pesticides were the problem, his findings "strongly supported such a hypothesis".
He cautioned: "Neurodevelopmental consequences of exposure to pesticides and nitrates many not be obvious for many decades."
Other work by the same team showed that more babies are born prematurely when pesticide use is at its highest.
Dr James Lemons, also of Indiana University, said: "I believe this work may lay the foundation for some of the most important basic and clinical research and public health initiatives of our time.
"To recognise that what we put into our environment has potential pandemic effects on pregnancy outcome and possibly on child development is a momentous observation, which hopefully will transform the way humanity cares for its world."
Pesticides are already known to cause thyroid problems in mothers-to-be - a condition thought to affect the intelligence of the unborn child.
Other studies have shown that exposure to pesticides almost doubles a person's risk of Parkinson's disease.
In 2005, the Royal Commission on Environmental Pollution said that spraying fields was a potential health risk and could be responsible for diseases including Parkinson's, cancer and ME.
Georgina Downs, of the UK Pesticides Campaign, said: "Pregnant women and their unborn babies, babies and young children and those who are ill already are particularly vulnerable to the effects of pesticides."
The Crop Protection Association, which represents pesticide manufacturers, says pesticides are vital to farming and gardeners and their use is strictly regulated.
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