Expectant mothers worried about their chances of having a premature baby may want to be more mindful of the weather.

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According to research, exposure to Extreme hot or cold conditions during pregnancy can put the unborn child at an increased risk for Preterm Birth.

“Our findings indicate that it may well be prudent to minimize the exposure of pregnant women to extremes in temperature,” said Pauline Mendola, researcher at the National Institutes of Health (NIH).

The first seven weeks of pregnancy are crucial, according to the study. It’s at this time that heatwaves or cold spells can lead to early delivery.

But for a woman exposed to extreme heat for most of her pregnancy, there were risks of preterm birth as well.

“[Researchers] theorized that, during cold spells, people are more likely to seek shelter and so could more easily escape the cold’s effects,” according to NIH. “But during extreme heatwaves, people are more likely to endure the temperature, particularly when the cost of or access to air conditioning is an impediment.”

More than 223,000 births at 12 clinical centers across the U.S. were reviewed for this study, which was published in Environmental Health Perspectives, a medical journal. Researchers compared the birth times to hourly temperature records in the region of each clinic.

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For women who experienced cold spells for the first seven weeks of pregnancy, there was a 20 percent increased risk of delivering their child before 34 weeks. If the first seven weeks occurred on hot days, there was an 11 percent increase in birth before 34 weeks.

Preterm birth, which is the birth of a child before 37 weeks, puts infants at risk for death or long-term disability.

“The researchers theorize that the stress of temperature extremes could hinder the development of