A study of more than 2.5 million pregnant people in California found that those exposed to wildfire smoke for at least one day faced a higher risk of giving birth prematurely.
The findings were presented Saturday at the Society for Maternal-Fetal Medicine’s annual meeting and are currently undergoing peer review. They're set to be published in the American Journal of Obstetrics and Gynecology.
The researchers collected data from hospital records of pregnant people from 2007 to 2012, then analyzed daily estimates of wildfire smoke in the participants' ZIP codes during their pregnancies, based on satellite images.
The results suggested that just one day of smoke exposure slightly raised the risk of spontaneous preterm birth — defined as before the 37th week of pregnancy. But the odds of preterm birth increased by 0.3% with each additional day of smoke exposure.
"Most pregnant persons are having well over one day of exposure, and the chronicity of this exposure, which continues to increase, is really the worrisome relationship with wildfire smoke," said Dr. Anne Waldrop, the study’s lead author and a maternal-fetal medicine fellow at Stanford University.
Waldrop's team estimated that 86% of the pregnant people studied in California had been exposed to fine particulate matter from wildfire smoke during their first or second trimesters, or during the four weeks leading up to conception. On average, the study participants were exposed to more than seven days of smoke during that time.
Exposure was only associated with preterm birth within the first 20 weeks of pregnancy, however, not before conception.
The term fine particulate matter refers to tiny particles in the air that are less than 4% of the diameter of a human hair. Once inhaled, the particles can penetrate into the lungs and enter the bloodstream. They can increase a person's risk of asthma, lung cancer and other chronic lung diseases, particularly among vulnerable groups like older people, infants and children, and those who are pregnant.