In a new study — the first of its kind — researchers fed water laced with fracking chemicals to pregnant mice and then examined their female offspring for signs of impaired fertility. They found negative effects at both high and low chemical concentrations, which raises red flags for human health as well.

“These are preliminary findings,” Susan Nagel, the study’s senior author and an associate professor in Obstetrics, Gynecology and Women’s Health at the University of Missouri School of Medicine, told me. “But I think they suggest that we should absolutely be looking more closely at the potential health effects (of these chemicals).”

Published in the September issue of Endocrinology, the study adds to a growing body of research on the human health effects of fracking chemicals and the potential hazards of related air and water contamination. Just earlier this year, Nagel co-authored another study that found water located downstream from a fracking wastewater site contained levels of endocrine-disrupting chemicals high enough to negatively effect the aquatic inhabitants.

In this most recent study, Nagel and colleagues selected 23 chemicals found in oil and gas operations, including fracking, and mixed them at four different concentrations to mimic what’s been found in drinking water and groundwater as well as in industrial wastewater. More specifically, the contaminated water samples were designed to reflect “environmentally relevant” concentrations, with the two lowest doses equivalent to what’s been reported in drinking water in drilling regions and the highest dose meant to mimic industrial wastewater. The contaminated water was then given to pregnant mice on day 11 of pregnancy through birth. The female offspring that were exposed to the chemicals in utero were then compared to female offspring that did not receive the contaminated water. The study noted that of the more than 1,000 chemicals used in unconventional oil and gas operations, at least 130 are known or suspected endocrine disrupters.

The researchers found that the exposed group presented notable differences in key hormones related to reproductive health, including disruptions in the development of the ovarian follicle, which is central to egg development. In particular, the female mice offspring exposed to smaller concentrations of the fracking chemicals had fewer ovarian follicles, which may suggest fewer eggs and shorter fertile periods. On the other end, the offspring exposed to high doses of chemicals experienced a follicle increase, which suggests “inappropriate follicle activation (or accumulation) and ultimate follicle death.”

In addition to reproductive health impacts, the exposed mice also had altered pituitary hormone production, increased body weights, and increased cardiac fibrosis (a thickening of the heart muscle and an indicator of future heart failure) as well as increased heart weights. (In a 2014 study, researchers found a relationship between proximity to natural gas wells and the prevalence of congenital heart defects in human babies.) Nagel told me that while the reproductive health findings don’t automatically mean the exposed offspring will have trouble getting pregnant or staying pregnant, “it is likely that suppressed reproductive hormones will alter fertility.” Nagel also noted that when it comes to the “developmental effects of endocrine disrupter exposure, mice have been very good predictors of human health.”

In terms of which specific chemicals might be responsible for the altered development in exposed mice, the researchers aren’t sure.

“We can’t say which chemical is the culprit,” Nagel said. “We’ll be dissecting the chemical question further in the future, but right know we don’t’ know. It’s definitely a chemical soup.”

Also published in September in the journal Fertility and Sterility was a systematic literature review on the relationship between oil and gas extraction and human reproduction. Co-authored by Nagel as well, she her colleagues — Victoria Balise, Chun-Xia Meng, Jennifer Cornelius-Green, Christopher Kassotis and Rana Kennedy — concluded:

The integration of the evidence suggests negative health impacts on reproduction from occupational and residential exposure to oil and gas extraction activities. Specifically, there is moderate evidence for increased risk of miscarriage, prostate cancer, birth defects, and decreased semen quality. However, no conclusions can be drawn for the following end-points. The evidence is low and inadequate for testicular, breast, or female reproductive cancers, birth outcomes associated with paternal exposures, and stillbirth. The evidence for both sex ratio and low birth weight is inconsistent.

There is ample evidence for disruption of the estrogen, androgen, and progesterone receptors with individual chemicals and complex mixtures of chemicals and waste products related to oil and gas extraction. These data provide a strong mechanistic rationale for how oil and gas activities may increase the health risks outlined herein.

To continue building on the mouse research, Nagel said she and colleagues plan to study the actual fertility of the female mice exposed to fracking chemicals prenatally (i.e., whether they can get and stay pregnant and if their number of offspring are affected) as well as follow up on the metabolic and heart findings. To request a full copy of the mouse study, visit Endocrinology.

Kim Krisberg is a freelance public health writer living in Austin, Texas, and has been writing about public health for nearly 15 years.