What causes so much preterm birth in Fresno County?

Fresno County, in the San Joaquin Valley of California (CA) has significantly higher levels of preterm birth than the rest of California (12.1% compared to 9.6% in CA in 2012)—but why? Preterm birth happens when babies are born too soon, before 37 weeks gestation, and factors including environmental pollution may contribute. We wanted to understand more about what might be causing the disparity seen in Fresno because preterm birth can have major and lifelong effects on a baby’s health.

Our study found bad news and good news. The bad news– pollution and social stressors, separately and in combination, are associated with increased preterm birth in Fresno. The good news: we might already have solutions on hand! A program aimed at supporting pregnant women is linked to significantly decreased preterm birth.

Fresno County has high levels of environmental pollution, including from air pollution, drinking water contaminants, pesticides, hazardous waste, and traffic. The Communities Environmental Health Screening Tool (also known as the CalEnviroScreen) was developed by CA’s Office of Environmental Health Hazard Assessment to identify communities disproportionately burdened by multiple sources of pollution. You can think of it like a ‘report card’ for the environment and vulnerability of different areas of California—the tool combines scores for pollution and social vulnerability factors like poverty into an overall score or grade. And, as shown in the map below, the area around Fresno County (which is roughly highlighted by the oval) stands out compared to the rest of the state (but unlike a report card a higher score is worse).

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CalEnviroScreen Scores of Pollution Burden by County in California (Red is more pollution; Green is less pollution)

Fresno County is a diverse area in terms of race/ethnicity and household income with a majority of the population being of non-white race and of lower income and a variety of environmental exposures, which may affect health outcomes including preterm birth. Our study used the information from CalEnviroScreen to look at how multiple environmental, social and medical factors are related to preterm birth in Fresno County from 2009 to 2012. In addition to looking at the factors individually, we also evaluated social factors like poverty and unemployment in combination with pollution to see if together they had an influence on preterm birth.

We analyzed 53,843 total births and found a strong association of preterm birth with total pollution– the chance of having a preterm birth was twice as likely for mothers exposed to high versus low levels of pollution. It was even more likely if they also lived in an area with lower income. Considering specific sources of pollution, preterm birth was more common in areas with more drinking water contaminants (such as uranium and trichloroethylene) and more diesel particles from cars and trucks. Trichloroethylene is currently under evaluation by US EPA. For all of these findings, we used statistical methods to control for other maternal factors that are known to affect preterm birth, like age, education and income.  Taken together, these findings indicate that both pollution and social stressors likely have important contributions to increased preterm birth rates in Fresno compared to the rest of CA.

But on the bright side, we found factors associated with decreasing preterm birth rates. Families participating in the Women Infants and Children (WIC) program were less likely to have a preterm birth even when they lived in areas of high pollution and low income. The WIC program provides access to supplemental foods, healthcare referrals and nutritional education for pregnant women. This is an example of a program that may be having a positive effect on reducing preterm birth in Fresno county, and our findings suggest that the addition of similar programs could be a positive intervention to reduce preterm birth in low-income areas.

Overall, we found multiple pollution exposures were related to increased preterm birth in Fresno County, and that the effect was stronger among the most disadvantaged communities. This supports other evidence finding environmental exposures are important to reduce if we want to lower the chances of babies being born preterm, and furthermore that this is especially important in areas with lower income and resources. For example, Casey and colleagues found a significant reduction in preterm births following removal of a major pollution source (specifically coal and oil power plants) from California communities.

What should be done now? We will continue to explore potential sources of pollution and their effects on preterm birth. But this data can also be used now to inform ongoing policy decisions, such as those to protect pregnant women and their children and to provide additional evidence to support the state’s actions to reduce air pollution. This study was able to look at many different pollutants and give us some clues as to where to look deeper to find more evidence of the harmful effects of pollution on our most vulnerable populations. And also, our study shows the importance of government programs to assist families in need so that they don’t have to suffer the additional burden of a preterm birth baby.


PadulaAmy Padula, PhD, is an Assistant Professor with the Program for Reproductive Health and the Environment in the Department of Obstetrics, Gynecology and Reproductive Sciences at the University of California, San Francisco. Her doctorate is in Epidemiology from the University of California Berkeley and her postdoctoral training was at Stanford University.  Her research has been on the effects of ambient air pollution during pregnancy on adverse birth outcomes including preterm birth, low birth weight and birth defects. The projects have expanded to evaluate social factors, comorbidities during pregnancy and gene-environment interactions. Dr. Padula has an K99/R00 Transition to Independence Award from the National Institute of Environmental Health Science and last year was named one of the 20 Pioneers under 40 in Environmental Public Health by the Collaborative on Health and the Environment.
Dr. Padula’s co-authors include Hongtai Huang, Rebecca Baer, Laura August, Marta Jankowska, Laura Jellife-Pawlowski, Marina Sirota and Tracey Woodruff.