Pregnancy stress is known to be exacerbated by summer heat. However, for many expectant women, extreme heat is considerably worse than a sweaty irritation.
According to recent studies, the risk of miscarriage increases significantly as the temperature rises. The results show that the chance of miscarriage is 44 percent higher in late August than in early February, for instance.
Over 12,000 women’s seasonal variations and pregnancy outcomes were examined by Wesselink and her coworkers. Late August saw a rise in the number of Spontaneous abortion rates especially for those living in the southern and midwestern United States.
“One of our hypotheses is that heat may trigger miscarriage, which is something that we are now exploring further,” says Amelia Wesselink, Ph.D., 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.”
Pregnancy Study Online (PRESTO), a web-based fertility study from the Boston University School of Public Health, tracked 12,197 American and Canadian women from 2013 to 2020 for up to a year. Participants in the study provided information on their lifestyle, income, education, race/ethnicity, and answers to follow-up questions regarding their pregnancies and/or miscarriages.
Non-Hispanic white persons made up the majority of the study’s participants (86%), and most had completed college (79 percent ). Nearly half had a yearly income of over $100,000. (47 percent ). The study did not include people who were looking for fertility treatments. In the first 12 months of trying to conceive, half of the women (6104) reported doing so, and nearly one in five (19.5%) of those who did miscarry.
The month with the lowest rate of lost pregnancies, late February, had a 44 percent lower chance of miscarriage than late August. This pattern was almost always observed in the first eight weeks of pregnancy.
For pregnancies of any stage, the risk of miscarriage increased by 31% in late August. With peaks in late August and early September, respectively, the South and Midwest experienced the highest rates of miscarriage in relation to excessive heat.
“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, Ph.D., 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, Ph.D., researcher at Johns Hopkins Bloomberg School of Public Health in Baltimore.
No analysis backed up the claim made in the paper that physical exercise may influence the frequency of miscarriages, according to Ahmed. Wesselink notes that because study participants tended to be white and wealthier than the overall population, the results may not be applicable to everyone.
The results may not apply to lower-income groups because socioeconomic position has a significant impact in environmental exposures, particularly heat, says Wesselink, even if the researchers found some commonalities between people with incomes above $100,000 and those who earned less.
Wesselink and her colleagues published their findings May 2 in the journal Epidemiology.
Parvathy Sukumaran 
