As if climate change wasn’t bad enough, a new study published Wednesday in Environmental Health Perspectives suggests it may increase the risk of expectant mothers having a premature delivery.

Researchers from the National Institutes of Health examined the medical records of over 200,000 women who had delivered a child at one of twelve hospitals or medical centers throughout the United States. After cross-referencing the women’s area of residence during their pregnancy with available weather data, they found a consistent pattern: Women who lived in places that experienced extremely hot or cold weather during the first seven weeks of their pregnancy had an increased risk of later delivering their children prematurely, or before 37 weeks.

Especially cold days were defined as any days that were in the 10th percentile or lower of an area’s average temperatures, while scorchers were defined as any days that exceeded the 90th percentile.

“Our findings indicate that it may well be prudent to minimize the exposure of pregnant women to extremes in temperature,” said study senior author Dr. Pauline Mendola, an epidemiologist in the Division of Intramural and Population Health Research at the NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), in a statement released by the NIH.

Specifically, Mendola and her team found that heat waves during the first 7 weeks of pregnancy were associated with a 20 percent increased risk of delivery before 34 weeks; a 9 percent increased risk of delivery during weeks 34 to 36; and a 3 percent increased risk of delivery during weeks 37 and 38. Cold snaps were associated with an 11 percent increased risk of delivery before 34 weeks, and a four percent increased risk during weeks 37 and 38.

Although experiencing both types of extreme weather early on appeared to be riskier for pregnant women, the heightened risk for hot days continued to exist until the 21st week of pregnancy. Similarly, women who lived in an area that experienced a rapid increase in temperature the week right before their pregnancy (5 degrees Fahrenheit or higher) also had a higher risk of premature delivery, though more so during the summer. Overall, the increased risk of premature delivery associated with hot weather ranged from 6 to 21 percent.

Though the current study can’t tell us why the risk is higher for hot days, the researchers speculated that pregnant women are less able to escape the heat than they are the cold, especially for those who can’t afford or access air conditioning. Another mystery is why this increased risk exists in the first place, though the researchers theorized that the stress of extreme weather can affect the blood flow between mother and child or otherwise interfere with fetal development. Children born prematurely are in turn at higher risk for a myriad of developmental and chronic conditions, such as autism and asthma. Heat waves and to a lesser extent cold snaps are already known to impact the health of vulnerable groups, such as the elderly or chronically ill.

Because climate change will and already has led to more extreme weather, the researchers advocate that more be done to protect mothers from these tumultuous temperatures.