"Preterm birth is a real public health problem," said John Meeker, who led the study. "We're not really sure how to go about preventing it, but this may shed light on environmental factors about which people may want to be informed."
Meeker, from the University of Michigan School of Public Health in Ann Arbor, added, "We knew that exposure to phthalates was extremely widespread in the US and possibly worldwide, and preterm births increased for unknown reasons over the past several decades."
Phthalates are included in products for a variety of reasons, including to make plastic flexible. Past studies have also found evidence that would suggest the chemicals may be tied to earlier births. Previous research has linked the chemicals to shorter pregnancies and lower birth weight.
"There are many possible routes of exposure depending on the chemical and the scenario," Meeker said. Most commonly, the chemical enters the body through food and beverages. It may also be absorbed through the skin. For the new study, the researchers used data from a study conducted at Brigham and Women's Hospital in Boston between 2006 and 2008.
Surveys and samples
During the study, pregnant women were asked to fill out surveys and provide urine samples throughout their pregnancies. The researchers compared 130 mothers who delivered their babies before 37 weeks to 352 women who delivered their babies at term. While each woman provided numerous urine samples during her pregnancy, the researchers analysed three to measure the amount of phthalates in their bodies.
They looked for breakdown products of a phthalate chemical known as DEHP. Overall, the two by-products MEHP and MECPP were more abundant in women who delivered their children early, compared to women who delivered after 37 weeks.
That was also true for MBP, a by-product of Dibutyk phthalate. According to the US Environmental Protection Agency, those chemicals are used to make products – such as plastic pipes, shower curtains and food packaging – soft and flexible. Each of the phthalates examined was linked to a risk increase of anywhere from 16% to 65% increase in risk for preterm birth.
It adds up
About one of every eight infants is born prematurely in the US, according to the Centres for Disease Control and Prevention. Shanna Swan, who wrote an editorial accompanying the new study in JAMA Paediatrics, said that difference may not mean much to an individual woman, but it adds up across a large population. Swan is a professor Department of Preventive Medicine at the Icahn School of Medicine at Mount Sinai in New York.
"There are a lot of indications or warnings that signal that women avoid phthalates when they can," she said. "I say 'when they can' because it's difficult. Most of the exposures are silent and we are not aware of them," she added. "We don't know how to avoid them. Previous studies have suggested that people who use fresh and organic produce – such as certain religious groups – have lower phthalate levels," Swan said. She added, however, that not many studies have examined the relationship between phthalates and preterm births.
Levels can change
Some studies, according to the researchers, have found no negative side effects from phthalate levels, but those only used levels from one urine sample. Phthalate levels can change during pregnancy.
Meeker said the new study can't prove higher phthalate levels caused women to deliver early or if they should stay away from the chemicals. "Our study wasn't really geared to look at that," he said. "Women may want to limit exposure if they can, but there are so many different points of exposure, which makes it difficult."