Pregnant women in a key US farm state are showing increasing amounts of a toxic weedkiller in their urine, a rise that comes alongside climbing use of the chemicals in agriculture, according to a study published on Friday.

The study, led by the Indiana University school of medicine, showed that 70% of pregnant women tested in Indiana between 2020 and 2022 had a herbicide called dicamba in their urine, up from 28% from a similar analysis for the period 2010-12. The earlier study included women in Indiana, Illinois and Ohio.

Notably, the study also found that along with a larger percentage of women showing the presence of dicamba in their bodies, the concentrations of the weed-killing chemical increased more than fourfold.

Both studies found that 100% of the women tested had 2,4-dichlorophenoxyacetic acid, better known as 2,4-D, in their urine; the more recent study showed detectable, but not significant, increases in concentration levels.

The findings add to a growing body of literature documenting human exposure to chemicals used in agriculture, and various known and potential health impacts. Many scientists have particular concerns about how farm chemicals affect pregnant women and their children, but say more research – and more regulatory scrutiny – is needed.

“These are two chemicals we’re concerned about because of their increasing use,” said Paul Winchester, a clinical professor of pediatrics at the Indiana University school of medicine who was not involved in this study.

Winchester called the findings “sobering” and said that there was growing evidence that these chemicals can be harmful to fetuses. “Fetal DNA is being shaped by these exposures,” he said. “What we’re seeing in other chemicals that have had longer pathways of study is that this is not benign exposure.”

Dicamba exposure has been linked to increased risk of liver and bile-duct cancers. Though the effects of 2,4-D on humans is less understood, some animal studies of 2,4-D exposure during pregnancy found low body weights and changes in behavior in the offspring, while other studies have found that exposure to 2,4-D appears to increase the risk of lymphoma.

Rising use

Gestational exposure to glyphosate – the key ingredient in the well-known Roundup herbicide – is associated with reduced fetal growth and other fetal problems. Glyphosate separately has been linked to cancer and other health problems.

Monsanto introduced glyphosate to agriculture decades ago, followed by genetically engineered crops created to withstand glyphosate spray, allowing farmers to kill weeds in their fields without harming the crops. That technology led to soaring use of glyphosate starting in the late 1990s, but its extensive and repeated use triggered an explosion of glyphosate-resistant weeds. Bayer, which acquired Monsanto in 2018, did not immediately respond to requests for comment.

Monsanto and other companies then developed dicamba- and 2,4-D-tolerant crops, and as a result, reliance on the two weedkillers has risen 10-fold since 2010.

The study notes that the vast majority of soybean and cotton seeds now sold and grown in the US are genetically engineered to tolerate a range of herbicides, including glyphosate, dicamba and 2,4-D.

“There’s no other way to explain a fourfold increase in dicamba in the urine of pregnant women in the midwest other than the planting of these technologies,” said Charles Benbrook, a co-author of the study and an expert witness for some cancer patients who have sued Monsanto, alleging Roundup caused their diseases. “This is a technology they probably should never have approved.”

Dicamba has particular volatility when sprayed on farm fields and is capable of traveling far from where it is sprayed, so exposure does not necessarily occur only on or near a farm where it is sprayed. Since dicamba-tolerant crops were introduced, the weedkiller has been documented on non-target crops, on trees miles from fields, even in rain.

Last week, a federal court in Arizona vacated the registrations of the weedkillers, ruling that the Environmental Protection Agency violated the law in approving dicamba by not giving adequate opportunity for public comment. It was the second time a federal court has banned dicamba.

But this week, the EPA said that despite the court ruling, it will still allow millions of gallons of dicamba to be sprayed this growing season.

This story is co-published with the New Lede, a journalism project of the Environmental Working Group

• This article was amended on 21 February 2024. An earlier version said that 2,4-D was dichloroacetic acid rather than dichlorophenoxyacetic acid.