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Environmental Factor, March 2013

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Study links prenatal exposure to PM with low birth weight

By Monica Frazier

Tracey Woodruff, Ph.D.

Woodruff and her team are dedicated to educating policymakers and clinicians about the effects of environmental exposures during windows of susceptibility in gestation and early childhood on the lifelong health of children. (Photo courtesy of UCSF)

A global team of researchers has reinforced concerns that exposure to increased levels of particulate matter (PM) throughout pregnancy can lead to an increased risk for low birth weight. Published online by Environmental Health Perspectives (EHP), this new research highlights the need for continued study of environmental exposures and their effects on the fetus.

The study, a meta-analysis headed by NIEHS grantee Tracey Woodruff, Ph.D., director of the Program on Reproductive Health and the Environment in the Department of Obstetrics, Gynecology, and Reproductive Sciences at the University of California, San Francisco (UCSF), and Jennifer Parker, Ph.D., from the Office of Analysis and Epidemiology at the National Center for Health Statistics, evaluated pregnancy outcomes based on levels of PM throughout pregnancy, as well as during each trimester.

“Low birth weight is a very important risk factor that is of general concern among clinical and public health audiences,” Woodruff explained, in an interview with EHP. “Babies who are born too small, for their gestational age, are at increased risk of a variety of morbidity outcomes, either during infancy or during childhood … Now we know, from very interesting research, that even the effects that occur early in gestation … can also be a marker for increased risk of adult disease.” 

With PM, size matters

PM is categorized by the U.S. Environmental Protection Agency as PM10 and PM2.5, based on size and associated potential toxicity. Sources of PM include cars, trucks, power plants, agriculture, and road dust.

The study is, to Woodruff’s knowledge, the largest meta-analysis of its kind, with more than 3 million births in 9 countries, accounted for by the efforts of 14 International Collaboration on Air Pollution and Pregnancy Outcomes centers. What Woodruff and colleagues have found is that pregnancies with elevated exposures to PM10 or PM2.5 are correlated with low birth weight and inversely correlated with normal birth weight.

In addition, the cohort identified factors from previous studies, including differences in how exposure is gauged, that may have contributed to previous discrepancy regarding PM’s role in birth weight.

Comparing births across the globe

Complicating factors in a study like Woodruff’s include how to normalize for factors such as socioeconomic status among nonhomogeneous populations. For example, populations living in areas with higher levels of particulates may also be more or less wealthy or more or less educated than populations in areas with lower levels of particulates. The researchers looked at the correlation between PM and low birth weight, with and without normalization for these factors, and saw higher particulate exposure associated with low birth weight in either case.

Interestingly, smaller sized particulates seem to play a larger role in birth weight than larger sized. These smaller particulates, which have the potential to be more invasive to the human body, are likely more toxic. Even with elevated levels of PM, the resultant risk of low birth weight is relatively modest, but, according to the researchers, it constitutes a significant public health concern because of the range of adverse health effects associated with low birth weight.

The mechanism of how exposure to particulates results in low birth weight is not clear, and will likely be an active area of study in the future. One focus Woodruff mentioned during her recent podcast interview involves the tendency of particulates to raise blood pressure, a side effect that combined with pregnancy could have adverse effects.

Citation: Dadvand P, Parker J, Bell ML, Bonzini M, Brauer M, Darrow L, Gehring U, Glinianaia SV, Gouveia N, Ha EH, Leem JH, van den Hooven EH, Jalaludin B, Jesdale BM, Lepeule J, Morello-Frosch R, Morgan GG, Pesatori AC, Pierik FH, Pless-Mulloli T, Rich DQ, Sathyanarayana S, Seo J, Slama R, Strickland M, Tamburic L, Wartenberg D, Nieuwenhuijsen MJ, Woodruff TJ. 2013. Maternal exposure to particulate air pollution and term birth weight: a multi-country evaluation of effect and heterogeneity. Environ Health Perspect; doi:10.1289/ehp.1205575 [Online 6 February 2013].

(Monica Frazier, Ph.D., is an Intramural Research Training Award fellow in the NIEHS Mechanisms of Mutation Group.)

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