Exposure to high temperatures during pregnancy is linked to slower thalamic growth in children

Exposure to high outdoor temperatures during pregnancy and early infancy appears to slow the growth of a specific brain region later in childhood. This suggests that rising global temperatures may have long-lasting effects on early brain development. The findings 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 age three, a child’s brain reaches about 80 percent of its adult size. During this time, the brain experiences intense synaptic proliferation, which is the rapid formation of connections between nerve cells.

The infant brain also exhibits high neural plasticity, meaning it is highly adaptable to new physical experiences and sensory information. This same adaptability makes the developing brain highly sensitive to environmental disruptions. A disruption during this window can cascade into later developmental stages.

Climate change is driving an increase in extreme weather across the globe, leading to a growing interest in how temperature changes affect human health. Previous studies tend to show links between early heat exposure and changes in human cognition and mental health. Few studies have examined the physical structure of the child brain to see if it changes in response to temperature.

A team of scientists from institutions in Spain, the Netherlands, and the United States designed the current study to fill this gap. They aimed to see if exposure to hot and cold temperatures from conception to age eight and a half provides evidence of physical brain changes during late childhood and early adolescence.

To explore this, the 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 tracking child development from fetal life onward. The mothers of these children were enrolled in the study between 2002 and 2006.

The team used a high-resolution climate model to estimate the outdoor temperatures around each child’s home. They calculated weekly average temperatures during the mother’s pregnancy and monthly average temperatures from the child’s birth until age eight and a half. This allowed them to map out 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 pictures of the internal organs. The children had their first scan at an average age of 10.1 years and a second scan at an average age of 14.0 years.

The scientists measured four global brain volumes, such as the total volume of cerebral white matter and cortical gray matter. They also measured seven deeper, subcortical structures, including the hippocampus, the amygdala, the caudate nucleus, the putamen, and the thalamus. They then analyzed whether the growth of these structures changed depending on the temperatures the children experienced early in life.

The data provides evidence for a specific window of vulnerability from conception through the fifth month of life. When children were exposed to higher temperatures during this period, they showed slower growth in the thalamus between ages 9 and 15. The thalamus acts as a central relay station in the brain, processing and transmitting sensory and motor information to the outer cerebral cortex.

The researchers compared a normal reference temperature of 12.5 degrees Celsius (about 54.5 degrees Fahrenheit) to a high exposure temperature of 20.5 degrees Celsius (about 68.9 degrees Fahrenheit). This high temperature represents the 95th percentile for the region, meaning it is warmer than 95 percent of the typical temperatures recorded there.

Exposure to this higher temperature during the first trimester of pregnancy was associated with a reduction in thalamic growth of 25.80 cubic millimeters. The authors noted that this reduction corresponds to a nearly seven-month delay in normal thalamic development.

Similar reductions were seen for exposure during other early life stages. The cumulative effect of heat exposure during the second trimester was a reduction of 25.60 cubic millimeters in thalamus growth. For the third trimester, the reduction was 22.92 cubic millimeters. Exposure during the first three months of life was linked to a reduction of 5.31 cubic millimeters.

No other brain structures showed this association, and cold temperatures did not seem to impact brain volume during the analyzed periods. The researchers suggest the thalamus may be especially sensitive because it is one of the first brain regions to develop during pregnancy, following a very tightly regulated developmental timeline. It also has a very rich blood supply in the fetal stage, which might make it highly sensitive to heat-induced changes in maternal blood flow or nutrient transfer.

Heat exposure during pregnancy can stress the maternal body, which tends to affect the fetus. High temperatures can alter placental function and increase maternal stress hormones like cortisol. Heat stress may also disrupt the signaling of serotonin, a chemical messenger that is highly involved in early thalamus development.

The team also looked at cognitive and behavioral data gathered from questionnaires. They found that slower thalamic growth tended to be associated with more externalizing symptoms in adolescents. Externalizing symptoms are outward negative behaviors, such as aggression, rule-breaking, or defiance. They did not find a link between thalamic growth and general intelligence or cognitive performance.

The current study provides a detailed look at environmental impacts on the brain, but it comes with several limitations. The researchers relied on outdoor temperatures at the children’s residential addresses. They did not have data on indoor temperatures, school environments, or how much time the children spent outside.

Air conditioning is uncommon in the Netherlands, meaning indoor temperatures often track closely with outdoor temperatures in the summer. Still, individual behaviors like using fans or seeking shade could not be factored into the analysis.

The researchers noticed a surprising trend where children living in neighborhoods with a higher socioeconomic status seemed to show greater vulnerability to heat. They suspect this might be due to housing differences. Homes in wealthier neighborhoods might be better insulated, which keeps heat trapped indoors during the summer.

Readers should avoid assuming that these exact temperatures will cause the same effects in entirely different climates. A temperature of 20.5 degrees Celsius is considered quite warm for the Netherlands, but populations in tropical or desert climates are biologically and culturally adapted to completely different temperature baselines. The study also cannot prove that early heat exposure directly causes behavioral problems, as the relationship between brain size, environment, and daily behavior is highly complex.

The research team only collected brain scans at two specific ages. This limits their ability to track more complex growth patterns, such as periods where the brain might catch up in its development after an early delay. More frequent brain scans would help clarify whether the thalamus remains smaller throughout adulthood.

Future research could explore whether this heat exposure contributes to specific neurodevelopmental conditions like autism spectrum disorder, which previous studies have linked to thalamus changes. Studies might also look at other temperature metrics, such as sudden heat waves or cold spells, rather than just monthly averages. Preventive measures, such as providing public cooling spaces or adjusting workplace policies for pregnant individuals, could be explored as ways to support early brain health.

The study, “Early life ambient temperature and brain volumes change throughout childhood,” was authored by Laura Granés, Esmée Essers, Michelle S.W. Kusters, Sami Petricola, Henning Tiemeier, Carles Soriano-Mas, Joel Schwartz, and Mònica Guxens.