Exposure to high outdoor temperatures during pregnancy and early infancy may slow the growth of certain brain regions from infancy onwards. This suggests that rising global temperatures may have long-term effects on early brain development. The results of this study were recently published in the journal environment international.
The human brain undergoes rapid growth and structural changes during fetal development and the first few years of life. By the age of three, a child’s brain reaches about 80% the size of an adult’s. During this period, the brain experiences intense synaptic proliferation. This is the rapid formation of connections between nerve cells.
Infant brains also exhibit high neuroplasticity. This means that they are highly adaptable to new physical experiences and sensory information. This same adaptive capacity makes the developing brain highly sensitive to environmental perturbations. Disruptions during this period can have cascades into later developmental stages.
Climate change is causing an increase in extreme weather events around the world, and there is growing interest in how changes in temperature affect human health. Previous research tends to show a link between early heat exposure and changes in human cognition and mental health. Few studies have examined whether the physical structure of children’s brains changes in response to temperature.
A team of scientists from institutions in Spain, the Netherlands, and the United States designed the study to fill this gap. They aimed to see whether exposure to high and low temperatures from pregnancy to age 8 1/2 years would provide evidence of physical brain changes from late childhood to early adolescence.
To investigate this, researchers looked at data from 3,251 children living in Rotterdam, the Netherlands. These children were part of the Generation R study, a large population-based birth cohort that tracks children’s development from the prenatal period. The mothers of these children participated in this study from 2002 to 2006.
The team used high-resolution climate models to estimate the outdoor temperature around each child’s home. They calculated the mother’s average weekly body temperature during pregnancy and the child’s average monthly body temperature from birth until age 8 1/2. This allowed us to plan exactly what temperatures each child was exposed to during early development.
To measure brain development, the children underwent magnetic resonance imaging scans at two different time points. Magnetic resonance imaging is a medical imaging technique that uses magnetic fields and radio waves to create detailed images of internal organs. Children had their first scan at a mean age of 10.1 years and a second scan at a mean age of 14.0 years.
The scientists measured four global brain volumes, including the total volume of cerebral white matter and cortical gray matter. They also measured seven deeper subcortical structures, including the hippocampus, amygdala, caudate nucleus, putamen, and thalamus. We then analyzed whether the growth of these structures changes depending on the temperatures children experience during childhood.
This data provides evidence of a specific period of vulnerability from pregnancy to the first 5 months of life. They found that when children were exposed to high temperatures during this period, their thalamus grew slower between the ages of 9 and 15. The thalamus acts as the brain’s central relay station, processing and transmitting sensory and motor information outside the cerebral cortex.
The researchers compared a normal reference temperature of 12.5 degrees Celsius (approximately 54.5 degrees Fahrenheit) to a higher exposure temperature of 20.5 degrees Celsius (approximately 68.9 degrees Fahrenheit). This maximum temperature ranks in the 95th percentile for the region, meaning it is warmer than 95 percent of the typical temperatures recorded there.
Exposure to this high temperature during the first trimester of pregnancy reduces thalamic growth by 25.80 cubic millimeters. The authors noted that this reduction corresponds to an almost 7-month delay in normal thalamic development.
Similar reductions were seen for other early-stage exposures. The cumulative effect of heat exposure during the second trimester of pregnancy was a 25.60 cubic millimeter reduction in thalamic growth. The loss during the third trimester was 22.92 cubic millimeters. Exposure during the first 3 months of life was associated with a reduction of 5.31 cubic millimeters.
No other brain structures showed this association, and cold temperatures did not appear to affect brain volume during the analysis period. The researchers suggest that the thalamus may be particularly sensitive because it is one of the first brain regions to develop according to a very tightly controlled developmental timeline during pregnancy. Also, because the blood supply is so abundant during fetal life, it may be highly sensitive to heat-induced changes in maternal blood flow and nutrient transfer.
Exposure to heat during pregnancy puts stress on the mother and tends to affect the fetus. High temperatures can alter placental function and increase maternal stress hormones such as cortisol. Heat stress can also disrupt the signaling of serotonin, a chemical messenger deeply involved in the early development of the thalamus.
The research team also looked at cognitive and behavioral data collected from questionnaires. They found that slower thalamic growth tended to be associated with more externalizing symptoms in adolescence. Externalizing symptoms are external negative behaviors such as aggression, rule-breaking, and defiance. They found no association between thalamic growth and general intelligence or cognitive ability.
Although this research examines the influence of the environment on the brain in detail, there are some limitations. The researchers relied on the outdoor temperature of the children’s residential address. There was no data on indoor temperatures, school environments or the amount of time children spent outdoors.
Air conditioning is not common in the Netherlands, so the indoor temperature is often about the same as the outdoor temperature in the summer. Still, individual behaviors such as using a fan or seeking shade could not be factored into the analysis.
Researchers noticed a surprising trend: children living in areas of higher socio-economic status seemed to be more vulnerable to heat. They believe this may be due to differences in housing. Homes in wealthier areas are better insulated and may be less likely to trap heat indoors during the summer.
Readers should avoid assuming that these exact temperatures will cause the same effects in completely different climates. Although a temperature of 20.5 degrees Celsius is considered very warm in the Netherlands, people living in tropical or desert climates are biologically and culturally adapted to completely different temperature standards. Also, because the relationship between brain size, environment, and daily behavior is very complex, this study cannot prove that early heat exposure directly causes behavioral problems.
The research team only collected brain scans at two specific ages. This limits the ability to track more complex patterns of growth, such as periods during which the brain may catch up developmentally after an initial lag. More frequent brain scans can help reveal whether the thalamus remains small into adulthood.
Future studies may investigate whether this heat exposure contributes to certain neurodevelopmental conditions, such as autism spectrum disorders, which previous studies have linked to changes in the thalamus. Studies could look at monthly averages as well as other temperature indicators, such as sudden heat waves or cold waves. Preventative measures such as providing public cooling spaces and adjusting workplace policies for pregnant people could be considered as ways to support early brain health.
The study, “Infancy ambient temperature and brain volume change throughout childhood,” was authored by Laura Granés, Esmée Essers, Michelle SW Kusters, Sami Petricola, Henning Tiemeier, Carles Soriano-Mas, Joel Schwartz, and Mònica Guxens.
