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A recent study published in Environmental Research suggests that extreme hot or cold temperatures during pregnancy may increase the the risk that infants born at term will be of low birth weight, according to a study of U.S. women by researchers at the National Institutes of Health.

When exposed to extreme cold during the entire pregnancy (or just during the second and third trimesters), this increased the risk for low birth weight. When exposed to atypically hot temperatures during the whole pregnancy, or just during the third trimester also increased this risk. The odds for low term birth weight were highest when the whole pregnancy was exposed to extreme temperatures.

Pauline Mendola, Ph.D., an epidemiologist in the Division of Intramural and Population Health Research at NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), and one of the study’s senior authors, stated that, “Until we can learn more, it makes sense to reduce the amount of time that pregnant women are exposed to extreme hot or cold weather.”

Low birth rate refers to when an infant is born at a weight of less than 5.5 lbs at birth. This may be because they were born prematurely, or they may be of a lower weight despite having completed the 37-40 weeks that is considered to be a term pregnancy. Others can be of a significantly lower weight due to a number of underlying health problems, such as an illness, infection, or a failure to grow in the womb. Compared to infants of an average weight, low birth weight infants may be at higher risk for infection and developmental delays.

In the current study, NICHD researchers linked medical records from 223,375 births at 12 U.S. clinical centers to hourly temperature records for the region surrounding each center. They defined extreme cold as below the 10th percentile of average temperatures for a region; extreme heat was defined as being above the 90th percentile.

Mothers who had been exposed to cold during either the second or third trimester were 18 to 21 percent more likely to have an infant be of low birth weight. When exposed to extended cold over the entire pregnancy, term infants were 257 percent more likely to be of low birth weight. Term infants exposed to heat in the third trimester were 31 percent more likely to be of low birth weight, and infants born after the entire pregnancy occurred during unusually hot temperatures were 249 percent more likely to be of low birth weight.

Researchers are unsure exactly why exposure to atypically hot or cold temperatures would affect birth weight in this way. The authors have noted that previous studies have suggested that heat exposure could affect birth weight by raising oxidative stress (toxic byproducts which are formed when oxygen interacts with cells and tissues) and thereby increasing inflammation. It is also theorized that such extremes in temperature may reduce blood flow to the uterus, which would subsequently deprive the infant of oxygen and nutrients hindering the placenta’s ability to remove fecal wastes.

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