July 15, 2022 – Summer heat is known to aggravate pregnancy fatigue. But for many pregnant women, sweltering temperatures are far worse than sweaty discomfort.
New research shows that the risk of miscarriage rises sharply as the mercury rises. At the end of August, for example, the risk of losing a pregnancy is 44% higher than in February, according to the results.
“One of our hypotheses is that heat can trigger miscarriage, which we’re currently exploring in more detail,” says Amelia Wesselink, PhD, assistant professor of epidemiology at Boston University School of Public Health, who led the study team. “Our next step is to dig into the engines of this seasonal model.
She and her colleagues analyzed seasonal differences and pregnancy outcomes for more than 12,000 women. Spontaneous abortion rates peaked in late August, especially for people living in the South and Midwest United States.
From 2013 to 2020, 12,197 women living in the United States and Canada were followed for up to 1 year using Pregnancy Study Online (PRESTO), an online fertility study from the Boston University School of Public Health . Study participants answered questions about their income, education, race/ethnicity, and lifestyle, as well as follow-up questions about their pregnancy and/or pregnancy loss.
Most of those studied were non-Hispanic white (86%) and had at least a college degree (79%). Almost half earned more than $100,000 a year (47%). Those who seek fertility treatments were excluded from the study.
Half of the women (6,104) said they had conceived in the first 12 months after trying to get pregnantand nearly one in five (19.5%) of those who conceived miscarried.
The risk of miscarriage was 44% higher at the end of August than it was at the end of February, the month with the lowest rate of lost pregnancies. This trend was observed almost exclusively for pregnancies in their first 8 weeks. The risk of miscarriage increased by 31% at the end of August for pregnancies at any stage.
The link between miscarriage and extreme heat was strongest in the South and Midwest, with peaks in late August and early September, respectively.
“We know so little about the causes of miscarriages that it is difficult to link the seasonal variation in risk to any particular cause,” says David Savitz, PhD, professor of epidemiology and obstetrics, gynecology and pediatrics. at Brown University in Providence, RI, who helped conduct the study. “Exposure varies by summer, including a lower risk of respiratory infection during the hot season, changes in diet and physical activity, and physical factors such as temperature and sunlight. “
But another expert warned that extreme heat may not be solely responsible for the miscarriage rates seen in the summer.
“You have to be careful when associating the summer months with miscarriage because women may pursue more outdoor activities during the summer,” says Saifuddin Ahmed PhD, a researcher at Johns Hopkins Bloomberg School of Public. Baltimore Health.
Although the article suggests that physical activity may play a role in the frequency of miscarriages, no analysis has supported this claim, says Ahmed.
Additionally, study participants were mostly white and tended to be wealthier than the general population, so the results may not apply to everyone, Wesselink says. Although the researchers found some similarities between participants earning more than $100,000 a year and those earning less, socioeconomic status plays an important role in environmental exposures — including heat — so results may not hold true among low-income populations, says Wesselink.
Wesselink and his colleagues published their findings May 2 in the journal Epidemiology.