low birth rates
September 30, 2015

How climate change is causing low birth weights

Climate change gets the blame for a lot of things: wildfires or drought, rising sea levels, disappearing coral, and vanishing fish stocks. But an extensive new study by the University of Utah has discovered a more surprising effect of climate change: a higher incidence of low birth weights.

The two-year project led by Utah geography professor Kathryn Grace examined the interaction between precipitation, temperature, and birth weight in 19 African countries.

In developing countries, low birth weight is the most reliable way to measure when a pregnancy has been negatively affected by an external factor. Low birth weight is defined by the World Health Organization as any baby born under 2,500 grams, about five and a half pounds.

Grace and her colleagues examined nearly 70,000 births in 19 African countries between 1986 and 2010. She then matched these births with seasonal rainfall and air temperatures, as well as variables relating to the mothers and their household, such as education level and whether the household had access to electricity.

The team calculated the average precipitation for a given month within 10 km of the child’s birth location for each month up to one year prior to birth. The values were then summed over each trimester. (Trimesters are the three periods of pregnancy.)

The study also calculated the maximum daily temperature for a given calendar day, again within 10 km of the birth location. The number of days in each birth month where the temperature exceeded 105 F and 100 F as the maximum daily temperature were then added up over trimesters.

Evidence and impact of climate change

Grace’s team found that a pregnant woman’s exposure to reduced precipitation and an increased number of very hot days did lead to lower birth weights.

“Our findings demonstrate that in the very early stages of intra-uterine development, climate change has the potential to significantly impact birth outcomes. While the severity of that impact depends on where the pregnant woman lives, in this case the developing world, we can see the potential for similar outcomes everywhere,” said Grace, according to a press release.

Low birth weight can lead to a higher risk of infant mortality and illness, and a greater chance of developing disabilities. Education levels and income are also lower for infants born with a lower weight.

In these developing countries, intensive care units and other healthcare provision are thin on the ground and usually lack the necessary funding. Consequently, the financial burden of a low birth weight infant can be very significant.

A combination of precipitation and temperature

This is the first time researchers have utilized fine-resolution precipitation and temperature data alongside birth data to analyze how weather impacts birth weight.

The results were clear. An increase in hot days above 100 F during any trimester corresponds to a decrease in birth weight. In fact, just one extra day with a temperature above 100 F in the second trimester corresponded to a 0.9 g weight decrease. This result corresponded with a larger effect when the temperature threshold was increased to 105 F.

Conversely, higher amounts of precipitation during any trimester resulted in larger birth weights. On average, a 10mm increase in precipitation during a particular trimester corresponds to an approximate increase in birth weight of around 0.3-0.5 g.

“While the results are dependent on trimester and location, the data shows that climate change – a combination of increased hot days and decreased precipitation – correlate to lower birth weights. . .We need to work faster and differently to combat the evident stresses caused by climate change,” said Grace.


Feature Image: Kathryn Grace