It is not known what, if any, harm the chemicals might cause but there has been speculation it could lead to allergies, miscarriage, abnormalities or even cancer.
One of the researchers told the scientific journal Reproductive Toxicology: “This is the first study to highlight the presence of pesticides associated with genetically modified foods in maternal, foetal and nonpregnant women's blood.”
Pete Riley, the director of GM Freeze, a group opposed to GM farming, described the research as “very significant”.
The Agriculture Biotechnology Council, which speaks for the GM industry, has questioned the reliability and value of the research.
Dr Julian Little, its chairman, said: “Biotech crops are rigorously tested for safety prior to their use and over two trillion meals made with GM ingredients have been safely consumed around the world over the past 15 years without a single substantiated health issue.”