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Exposure to air pollution was associated with an increased risk of spontaneous preterm birth, according to a population-based cohort study of more than 400,000 births at one major California health system.

Spontaneous preterm birth positively correlated with exposure to particulate matter 2.5 μm or less in diameter (PM_2.5) (adjusted odds ratio [aOR] per IQR increase 1.15, 95% CI 1.12-1.18), as estimated by maternal residential histories, reported Jun Wu, PhD, of the University of California Irvine, and colleagues. The IQR was 2.76 μg/m³.

The excess risk of spontaneous preterm birth was arguably higher during the second trimester (aOR for total PM_2.5 concentration 1.10, 95% CI 1.08-1.12) compared with the first (aOR 1.07, 95% CI 1.05-1.09) and third trimesters (aOR 1.09, 95% CI 1.07-1.11), they wrote in JAMA Network Open.

Additionally, factors associated with disproportionately higher risk of preterm birth were lower education, lower income, limited exposure to green space, more exposure to wildfire smoke, and more exposure to extreme heat.

As such, lead study author Anqi Jiao, BS, a PhD candidate at the University of California Irvine, told MedPage Today that these findings suggest “a critical need for targeted public health interventions aimed at reducing the spontaneous [preterm birth] burden and promoting health equity among vulnerable groups.”

The associations “with regards to the intersection of race, neighborhoods, environmental factors, and obstetric outcomes” are all plausible, commented David Hackney, MD, a maternal-fetal medicine specialist at Case Western Reserve University in Cleveland, who was not part of the study.

However, he warned that “one of the pitfalls of using enormous datasets and advanced statistical methods is that you identify associations which are likely true, and formally meet the criteria for statistical significance, but are so small that it is unclear how impactful they really are.”

Nevertheless, “any increased odds of [preterm birth] is potentially meaningful given the outcome, and the fact that rates have proved so incalcitrant against lowering,” said Hackney.

Previous research had already linked poor air quality to a slew of adverse health impacts, like increased risk of breast cancer, with exposure during pregnancy tied to increased risk of cerebral palsy in offspring.

In the present report, Wu’s group detected positive associations between spontaneous preterm birth and four out of five individual constituents of PM_2.5:

• Black carbon (aOR 1.15 per IQR increase, 95% CI 1.11-1.20; IQR, 1.05 μg/m³)
• Nitrate (aOR 1.09, 95% CI 1.06-1.13; IQR, 0.93 μg/m³)
• Sulfate (aOR 1.06, 95% CI 1.03-1.09; IQR, 0.40 μg/m³)
• Organic matter (aOR 1.05, 95% CI 1.02-1.08)

The relationship with ammonium exposure did not meet criteria for statistical significance (aOR 1.03, 95% CI 1.00-1.06).

In the future, Jiao said that the team will investigate the impacts of wildfire-emitted pollutants including PM_2.5 and polycyclic aromatic hydrocarbons on pregnancy outcomes to address this double jeopardy of risk.

The researchers had performed a retrospective study that included all singleton live births from 2008 through 2018 recorded within Kaiser Permanente Southern California, with accompanying data on the mother’s residence during pregnancy.

A model provided daily total PM_2.5 concentrations during 2007 to 2018 at the census tract level. Researchers also turned to a geoscience-derived database of monthly concentrations of the five PM_2.5 constituents during 2007 to 2017 at a spatial resolution of 1 km × 1 km.

The study cohort ultimately included 409,037 births. Mothers averaged 30.3 years of age at delivery. By race, 51% were Hispanic, 26% were white, 12% were Asian, and 8% were Black.

Spontaneous preterm births — defined as birth after 20 weeks’ but before 37 weeks’ gestation, following spontaneous onset of labor, with no indication of concomitant pregnancy complications — constituted 4.73% of the births, based on an algorithm that extracted information about preterm labor triage and evaluation from hospital records.

Average exposure to total PM_2.5 during pregnancy was 11.40 μg/m³ and 11.54 μg/m³, respectively, among all births and spontaneous preterm births.

Mothers with spontaneous preterm birth tended to be older (age at least 35 years), self-identify as Black or Asian, have a lower educational attainment, be overweight, have pregestational diabetes and hypertension, and have a history of preterm birth.

Wu’s group reported that the overall study results were consistent across sensitivity analyses, such as when restricting the population to only consider each participant’s first birth.

Notably, the study was limited by “inevitable” exposure misclassification because the authors had to use census tract-level data and could not track personal time-activity patterns. They also only considered five major PM_2.5 constituents, relied on snapshot street view green space data, and did not assess different clinical phenotypes of spontaneous preterm birth.