Common weed killer may affect hormones tied to pregnancy and fetal development, new research finds

Exposure to glyphosate, the active ingredient in many weed killers, was linked to changes in several hormones that support pregnancy and fetal development-in one of the few studies to examine how a widely used herbicide may affect the body during pregnancy. The results come from a new study led by University of Michigan School of Public Health researchers.

The study, published in the Journal of Exposure Science and Environmental Epidemiology, looked at 752 pregnant women in Puerto Rico.

The research team found that higher levels of glyphosate and a related compound were associated with changes in hormones that help support the placenta, fetal growth, and timing of labor. Glyphosate is widely used in farming, landscaping and home weed-control products, and people can be exposed through food, water, soil, pesticide drift, or contact with treated areas.

“This is the most extensively used herbicide in the world, yet there are shockingly few research studies on the potential impacts it may have on human reproductive health, pregnancy, or fetal and child development,” said John Meeker, professor of Environmental Health Sciences at the University of Michigan School of Public Health and the paper’s senior author. “When considering our new findings along with those from experimental studies, it’s clear that increased attention should be paid to these potential risks and more research is desperately needed.”

Exposure was common among study participants

The researchers used data from the PROTECT birth cohort, a long-running study of environmental exposures and maternal and child health in Puerto Rico.

Participants gave urine samples up to three times during pregnancy, at about 18, 22 and 26 weeks. Researchers also measured several hormones in blood samples, including estrogen, thyroid hormones and corticotropin-releasing hormone, or CRH. CRH is a hormone involved in the body’s stress response and in biological processes related to labor.

Glyphosate and AMPA—the main substance glyphosate breaks down into in the environment—were found in most (70%) samples at study visits, showing that exposure was common among the participants.

For every moderate increase in AMPA levels, researchers found 10.6% lower estriol levels during pregnancy. A similar increase in glyphosate was linked to 8.3% lower estriol levels. AMPA was also linked to higher levels of the thyroid hormone T3.

Later in pregnancy, AMPA was linked to higher thyroid-stimulating hormone, and glyphosate was linked to higher CRH.

“The exposure levels we measured in Puerto Rico are higher than those reported for the US general population, and US territories historically bear a disproportionate share of environmental burdens,” said Mislael Valentín-Cortés, the study’s first author and a postdoctoral research fellow in the University of Michigan’s Department of Epidemiology. “Critically, these chemical exposures overlap with extreme weather events, disasters, and infrastructure failures, and their compounding adverse effects on health require more sustained attention than they currently receive.”

Hormones offer clues about possible health pathways

A handful of recent human studies have linked glyphosate exposure with some pregnancy-related outcomes, including preterm birth and differences in fetal growth. Until now, less was known about the biological changes that might help explain those associations.

This study points to hormone disruption as one possible pathway. The findings build on lab and animal studies suggesting that glyphosate-based herbicides may affect the endocrine system and add new human evidence from repeated measurements during pregnancy.

Researchers caution against overinterpreting results

The authors noted that the study was observational, meaning it can show links but cannot prove cause and effect on its own without additional considerations. Urine samples also reflect recent exposure and may not capture a person’s total exposure throughout pregnancy. The study did not test whether the hormone changes led to birth outcomes or later child health effects, but the researchers plan to explore this in the future with the collection of more data.

Even so, the researchers say the findings add to growing interest in how common environmental exposures may affect health during pregnancy.

“As public attention continues to grow around food, pesticides and environmental health, these findings help move the conversation from general concern to measurable changes in physiological processes,” Meeker said. “Understanding how common exposures may affect pregnancy is an important step toward protecting maternal and child health.”

Reference:

Valentín-Cortés, M.A., Cathey, A.L., Jenkins, H.M. et al. Glyphosate exposure and hormonal disruption in pregnancy: evidence from a birth cohort in Puerto Rico. J Expo Sci Environ Epidemiol (2026). https://doi.org/10.1038/s41370-026-00902-6