“There’s a lot we don’t know about preterm birth and we know even less about the disparities in those births.”

Those are words from Ondine von Ehrenstein, an assistant professor in the Department of Community Health Sciences at the UCLA Fielding School of Public Health, who recently examined the links between occupational exposures and preterm birth rates among Hispanic women. Perhaps not surprisingly to those in the public health world, von Ehrenstein and her research colleagues did find that Hispanic women are at particular risk for preterm birth associated with certain occupational conditions. However, what may be more surprising — and what could be critical to an effective intervention — was that the effect of occupational exposure on preterm birth risk varied by nativity, or whether a woman was U.S.-born or foreign-born.

“We found higher risks related to physically demanding and shift work, especially among U.S.-born Latina women,” von Ehrenstein told me. “Why is there such a difference? We’ll need further research to really understand.”

However, Ehrenstein and fellow researchers have begun to lay the groundwork. To conduct the study, which was published in the February issue of the American Journal of Public Health, researchers examined a case-control study nested within a cohort of more than 58,300 births in Los Angeles County in 2003 and used the U.S. Census Occupation Codes and Classification System to characterize the mothers’ occupations during pregnancy. The study noted that while previous U.S.-based studies have been conducted on preterm birth and occupational exposures (which is defined as both working conditions, such as daily working hours, and environmental exposures, such as indoor air quality), they have not teased out the relationship of the mother’s place of birth.

Authors also note that rates of preterm birth are higher in the U.S. than in many peer nations. (A recent March of Dimes report found that the U.S. preterm birth rate dropped to 11.5 percent in 2012 — a 15-year low — but it’s still the highest rate among industrialized nations.) Study authors von Ehrenstein, Michelle Wilhelm, Anthony Wang and Beate Ritz write:

The prevalence of preterm birth in the United States increased over the past decades with persistent racial/ethnic disparities, and it remains the main cause of infant mortality. Hispanic women have on average a higher prevalence of preterm birth than non-Hispanic White women, and risks tend to vary by nativity (US-born vs. foreign-born). These disparities remain poorly understood and have not to date been explained by social or demographic factors. …Occupational exposure may increase risks for preterm birth by interrupting the prenatal neuroendocrine balance, thereby promoting premature parturition (birth), and these adverse occupational influences may possibly affect Hispanic populations in the United States disproportionally and may possibly also be modified by nativity.

The study found that among all women who worked during pregnancy, there was a suggestion of increased risk of preterm birth among health care practitioners and technical occupations, building and grounds cleaning and maintenance, and food preparation and serving jobs. Risks were also associated with physically demanding work as well as shift work. When zeroing in on Hispanic women who worked while pregnant, the risk was higher among health care practitioners and technical occupations.

The risk of preterm birth also increased among Hispanic women doing shift work and physically demanding jobs — however, that risk only increased among Hispanic women who were born in the U.S. and not among foreign-born Hispanic women. The elevated odds ratio for preterm birth was statistically significant for foreign-born Hispanic women working in building and grounds cleaning and maintenance (though von Ehrenstein said it’s important to note that this group of Hispanic women also represented the vast majority of such workers). Still, foreign-born Hispanic women working in such maintenance jobs faced a preterm birth risk two-and-a-half times larger than women working in the office and administrative support sector.

“These ethnic disparities indicate that there is room for prevention,” von Ehrenstein said.

Researchers hypothesized a number of reasons for the differences. For example, the pronounced risk among Hispanic health care workers might indicate that such workers are “doing especially heavy physical work, although we currently have no evidence for this,” the study stated. Authors also noted that “lower acculturation has been reported for foreign-born compared with US-born Hispanics, and it has been associated with better support networks, healthier nutrition, and lower rates of smoking and alcohol consumption, reflecting healthier practices among women in the country of origin.” However, Ehrenstein pointed out that some of the biggest factors influencing pregnancy outcomes may be exposures that occur early in life — what she called the life course perspective — which could help explain the differences by nativity.

von Ehrenstein said the findings on nativity could be especially helpful when developing interventions. She also said that working to improve birth outcomes and infant health today — whether through policy or program — can reverberate throughout the lifespan.

“Early health risks may have lifelong implications,” von Ehrenstein said. “So if we want to reduce health costs and the implications for public health, a prime venue for prevention is really in promoting healthy pregnancies.”

To access the full study, click here.

Kim Krisberg is a freelance public health writer living in Austin, Texas, and has been writing about public health for more than a decade.