Led by University of Montreal researchers, the study has found that the risks of a premature birth quadruple if flaxseed oil is consumed in the last two trimesters of pregnancy.
A majority of pregnant women prefer to use natural health products during the pregnancy.
The most consumed natural health products by pregnant women are chamomile, green tea, peppered mint and flaxseed oil.
"We believe these products to be safe because they are natural. But in reality, they are chemical products and we don't know many of the risks and benefits of these products contrarily to medication," said Professor Anick Berard of the Universite de Montreal's Faculty of Pharmacy.
In the study involving 3354 Quebec women the only one product had a very strong correlation with preterm births was flaxseed oil.
"In the general population, the average rate of premature births is 2 to 3 percent. But for women consuming flaxseed oil in their last two trimesters that number jumps up to 12 percent," said Berard.
"It's an enormous risk," Berard added.