Summer heat is notorious for making the strain of pregnancy worse. But for many pregnant people, sweltering temperatures are much worse than a sweaty annoyance.
New research shows that the risk of miscarriage rises sharply as the mercury climbs. In late August, for example, the risk of losing a pregnancy is 44% higher than in February, according to the findings.
“One of our hypotheses is that heat may trigger miscarriage, which is something that we are now exploring further,” says Amelia Wesselink, PhD, an assistant professor of epidemiology at Boston University School of Public Health, who led the study team. “Our next step is to dig into drivers of this seasonal pattern.”
She and her colleagues analyzed seasonal differences and pregnancy outcomes for over 12,000 women. Spontaneous abortion rates peaked in late August, especially for those living in the southern and midwestern United States.
Spontaneous abortion was defined as miscarriage, chemical pregnancy (a very early miscarriage where the embryo stops growing), or blighted ovum (the embryo stops developing or never develops).
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 internet-based fertility study from the Boston University School of Public Health. Those in the study answered questions about their income, education, race/ethnicity, and lifestyle, as well as follow-up questions about their pregnancy and/or loss of pregnancy.
Most of the people studied were non-Hispanic white (86%) and had at least a college degree (79%). Almost half earned more than $100,000 annually (47%). Those seeking fertility treatments were excluded from the study.
Half of the women (6104) said they conceived in the first 12 months of trying to get pregnant, and almost 1 in 5 (19.5%) of those who conceived miscarried.
The risk of miscarriage was 44% higher in late August than it was in late February, the month with the lowest rate of lost pregnancies. This trend was almost exclusively seen for pregnancies in their first 8 weeks. The risk of miscarriage increased 31% in late 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 miscarriage that it’s difficult to tie seasonal variation in risk to any particular cause,” says David Savitz, PhD, a professor of epidemiology and obstetrics, gynecology, and pediatrics at Brown University in Providence, Rhode Island, who helped conduct the study. “Exposures vary by summer, including a lower risk of respiratory infection in the warm 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 the only culprit in summer’s observed miscarriage rates.
“You need to be careful when linking summer months to miscarriage, as women may pursue more outdoor activities during summer,” says Saifuddin Ahmed, PhD, a researcher at Johns Hopkins Bloomberg School of Public Health in Baltimore.
Although the paper suggested physical activity may play a role in miscarriage frequency, no analysis supported this claim, Ahmed says.
Also, participants in the study were mostly white and tended to be wealthier than the general population, so the findings may not apply to everyone, Wesselink says. Although the researchers saw some similarities between participants with income above $100,000 a year and those who earned less, socioeconomic status plays an important role in environmental exposures — including heat — so the results may not hold among lower-income populations, Wesselink says.
Wesselink and her colleagues published their findings May 2 in the journal Epidemiology.
Savitz, Wesselink, and Ahmed report no relevant financial conflicts of interest.