Depending on the individual phthalate, women with the highest levels of exposure during pregnancy had two-to-five times the odds of preterm birth, compared to women with the lowest exposure.
Before interventions can be implemented, however, more must be known about how phthalates interact with pregnancy, their potential association with women delivering early, and where exposures are coming from, the researchers caution.
"Preterm birth is a major public health challenge. Rates are significantly higher than they were 20 years ago and we don't know why. Other interventions have had limited effectiveness, and this helps shed light on a potential for environmental influences," said John Meeker, associate professor of environmental health sciences and associate dean for research at the U-M School of Public Health, who is the principal investigator of the study. "Next, we need to look at how pregnant women are exposed, and at what stage of pregnancy, to help inform exposure and risk prevention strategies."
Phthalates are used to make plastic materials more flexible and as solvents in personal care products such as perfumes, deodorants and lotions. They are found in food, adhesives, vinyl flooring, plastic shower curtains, some medications and more.
Meeker and colleagues studied 482 individuals selected from a larger population of pregnant women who delivered at the Brigham and Women's Hospital from 2006-08. For each woman, phthalate levels were measured in urine samples taken from up to three time points during pregnancy. The 130 mothers who delivered prior to 37 weeks showed significantly higher concentrations of four of nine phthalate metabolites that were measured in the study.
Dr. Thomas McElrath, a physician-scientist and key co-investigator on the study, developed and leads the original cohort study upon which this work is based in the Department of Obstetrics and Gynecology at Brigham and Women's Hospital.
"This is by no means the cause of premature birth, but it is a possible contributor—which is important because it is potentially modifiable," McElrath said. "This finding may be dramatic but women should not be alarmed. This is only the first step in a long research process that will be needed to clarify and confirm these results. It is simply too early to suggest making changes in prenatal care based on this study."
Kelly Ferguson, doctoral student at the U-M School of Public Health and first author of the paper, says the team's research provides the largest study of this relationship to date, and further improves on other studies by using multiple exposure measurements per subject.
"We have some ideas on how phthalates could cause preterm birth, although the exact mechanism is still unknown," she said. "One possibility we are considering is that phthalates are causing changes in oxidative stress or inflammation in the mother, and that these changes are leading to early labor."
The researchers say current studies remain inconclusive in assessing whether avoiding perfume, deodorant and other personal care products, and eating more fresh foods that undergo less processing and packaging, could lower women's phthalate exposure levels.
"Once we know more about pathways of exposure and mechanisms that cause this to happen, then it will be the time for interventions and policy action at the individual, clinical and federal levels," Meeker said. "These are things we are actively researching."
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