Extreme heat may increase miscarriage risk, study shows

A recent study found the risk for miscarriage was higher during summer months, leading researchers to suggest a connection between pregnancy loss and extreme heat.

Up to 30% of pregnancies end in miscarriage, which “can cause substantial morbidity,” including posttraumatic stress, depression and anxiety, researchers wrote in Epidemiology.

Little is known about what causes miscarriages, and identifying risk factors “is an important public health goal,” Amelia K. Wesselink, MPH, a research assistant professor of epidemiology at the Boston University School of Public Health, and colleagues wrote.

“Few studies have examined the association between heat and miscarriage risk, so this is definitely a topic that warrants further exploration,” Wesselink said in a press release.

The researchers conducted a web-based preconception cohort study known as Pregnancy Study Online, in which participants completed a baseline questionnaire that gathered information on demographics, medical histories and more. The researchers then sent participants follow-up questionnaires every 2 months for 1 year to keep up with pregnancy status.

The majority of participants were white (86%) and had earned a higher education degree (79%). They were located in all 50 states and 10 Canadian provinces.

Over 77,401 gestational weeks, 19.5% of 6,104 participants reported a miscarriage, according to the researchers.

Wesselink and colleagues found that participants had a 44% higher risk for an early miscarriage during summer months compared with in February. They also reported a 31% higher risk for miscarriage during any week of pregnancy in late August compared with February.

“Any time you see seasonal variation in an outcome, it can give you hints about causes of that outcome,” Wesselink said in the release. “We found that miscarriage risk, particularly risk of ‘early’ miscarriage before 8 weeks of gestation, was highest in the summer. Now we need to dig into that more to understand what kinds of exposures are more prevalent in the summer, and which of these exposures could explain the increased risk of miscarriage.”

The association between seasonality and miscarriages was stronger for people who face the hottest summers — those in the South and Midwest. This finding indicates that “early reproductive events may be susceptible to seasonally varying environmental or lifestyle exposures,” the researchers wrote.

In analyses adjusted for the month people began trying for pregnancy, miscarriage risk peaked in late August, according to Wesselink and colleagues. The adjusted peak/low ratio was 1.3 (95% CI, 1.1-1.6). This, along with the data showing stronger seasonal variation for women living in the South and Midwest, “further suggests that heat may play a role in the etiology of spontaneous abortion,” the researchers wrote.

Previous data have shown an association between extreme heat and sperm DNA fragmentation — potentially a link to miscarriage risk, according to the researchers. Additionally, maternal heat exposure in early pregnancy has previously been associated with birth defects. Therefore, the researchers wrote, “exposure to heat during early development could be teratogenic and lead to spontaneous abortion.”

Still, Wesselink and colleagues haven’t ruled out potential behavioral responses to heat such as physical activity or changes in diet that could “mediate the relationship with spontaneous abortion.” They also said that “exposures with a seasonal pattern could improve survival of afflicted embryos very early in gestation, allowing them to survive long enough to be detected, which would result in an apparent association between the exposure and spontaneous abortion.”

“We know that heat is associated with higher risk of other pregnancy outcomes, such as preterm delivery, low birth weight, and stillbirth, in particular,” Wesselink said in the release. “Medical guidance and public health messaging — including heat action plans and climate adaptation policies — need to consider the potential effects of heat on the health of pregnant people and their babies.”

Organizations such as the AMA have echoed this sentiment, declaring climate change a public health crisis and advocating for policies that help limit its effects.

References:

• Risk of miscarriage may increase during the summer. https://www.eurekalert.org/news-releases/957188. Published June 27, 2022. Accessed Jun 30, 2022.
• Wesselink AK, et al. Epidemiology. 2022;doi:10.1097/EDE.0000000000001474.