Climate change is a massive public health threat, with increasing temperatures being linked to deteriorating health, especially in vulnerable populations and including pregnant women and children.
Scientists have also previously shown that heat exposure increases the risk of preterm birth and stillbirth, while ongoing research continues to uncover worrying links to poor outcomes for mothers and their babies – congenital abnormalities, hypertension in pregnancy and low birthweight being some of the dangers of increasing heat.
One area that has not received as much attention is the long-term effect that heat exposure during pregnancy might have on the baby, write Matthew Chersich, Darshnika Pemi Lakhoo and Nicholas Brink in The Conversation, after conducting a systematic review of all the existing research on the this, and the socioeconomic consequences in later life.
They write:
Systematic reviews are designed to provide the highest level of medical evidence, collating and summarising all the findings of qualifying research, rather than relying on just one study.
Our findings were clear. They showed that people who had been exposed to excessive heat before birth suffered alarming lifelong effects.
The most common measure of heat is the average air temperature, but some studies used more complex measures that adjusted for the humidity, and other factors that influence how a person experiences heat.
How we define dangerous levels of heat for pregnant women is an ongoing focus of our research. The most likely scenario is that it is influenced by location, context and individual vulnerabilities. Different conditions may also have different harmful thresholds and periods of susceptibility.
We found 29 studies covering more than 100 years, allowing us to see effects throughout an individual’s lifespan. Some studies followed pregnancies closely to observe any ill-effects on the child. Others relied on population registries that recorded date and place of birth, allowing researchers to estimate the individual’s in-utero heat exposure.
More than 60% of studies were conducted in high-income countries in the global north, which often have cooler climates. This research relied on observing naturally occurring differences in heat exposure, rather than controlled trials.
Despite those research limitations, we found the majority of studies linked harmful long-term effects with increased heat exposure during pregnancy.
In particular, we found associations with worse educational performances and lower income in later life.
For example, in the US, annual income at the age of 30 was reduced by $56 (2008 equivalent) for every additional day with temperatures above 32°C during the first trimester of the mother’s pregnancy.
We also found harmful health effects that included increased risk of heart disease and hypertension, as well as childhood asthma and pneumonia.
Childhood pneumonia risks were estimated to increase by 85% for every degree Celsius increase in temperature over the course of the pregnancy. In Africa, the risk of malnutrition in children went up with increasing heat exposure in pregnancy. In the US, one study found a link with increasing risk of obesity.
Many studies also showed links to mental illnesses, including increased risk of eating disorders and schizophrenia. In fact, previous research has suggested links between the seasons and months in which a baby is born, with the risk of some mental illness. Our research suggests heat exposure could be one of the reasons behind this.
These effects seem to culminate in an association with lower life expectancy, where people who had been exposed to increased heat while in the womb were found to die younger.
We also found that the effects seemed worse for female foetuses in studies exploring sub-group vulnerabilities.
Multiple pathways
Understanding how and why these effects might be seen across completely different body systems was an important part of our research. We drew on our team of experts in human development, on research being conducted into the direct effects of heat on pregnant women, and on animal studies.
We propose that the effects of heat in pregnancy on the unborn baby are likely to occur through multiple pathways, including:
• worsening the health of the mother through illnesses like pre-eclampsia and diabetes;
• directly affecting the baby’s development, especially the nervous system (heat can cause birth defects);
• increasing the risk of preterm birth and other problems at the time of birth;
• directly changing the unborn baby’s DNA. This is likely to occur through changes in the epigenetic signature, an evolutionary mechanism that allows us to rapidly adapt to our environment by switching genes on and off.
One study even noted shortening of the unborn baby’s telomeres, the biological clock in our DNA that is linked to our limited lifespan.
There is an urgent need to conduct more research into this area and explore how and why these effects occur.
Call to action
Although the research is limited, our findings are worrying and support immediate individual, community and global action to protect pregnant women and their unborn babies from heat.
Study details
Impacts of heat exposure in utero on long-term health and social outcomes: a systematic review
Nicholas Brink, Darshnika Lakhoo, Matthew Chersich, & Climate and Heat-Health Study Group
Published in BMC Pregnancy & Childbirth on 4 May 2024
Abstract
Background
Climate change, particularly global warming, is among the greatest threats to human health. While short-term effects of heat exposure in pregnancy, such as preterm birth, are well documented, long-term effects have received less attention. This review aims to systematically assess evidence on the long-term impacts on the foetus of heat exposure in utero.
Methods
A search was conducted in August 2019 and updated in April 2023 in MEDLINE(PubMed). We included studies on the relationship of environmental heat exposure during pregnancy and any long-term outcomes. Risk of bias was assessed using tools developed by the Joanna-Briggs Institute, and the evidence was appraised using the GRADE approach. Synthesis without Meta-Analysis (SWiM) guidelines were used.
Results
A total of 18 621 records were screened, with 29 studies included across six outcome groups. Studies were mostly conducted in high-income countries (n = 16/25), in cooler climates. All studies were observational, with 17 cohort, 5 case-control and 8 cross-sectional studies. The timeline of the data is from 1913 to 2019, and individuals ranged in age from neonates to adults, and the elderly. Increasing heat exposure during pregnancy was associated with decreased earnings and lower educational attainment (n = 4/6), as well as worsened cardiovascular (n = 3/6), respiratory (n = 3/3), psychiatric (n = 7/12) and anthropometric (n = 2/2) outcomes, possibly culminating in increased overall mortality (n = 2/3). The effect on female infants was greater than on males in 8 of 9 studies differentiating by sex. The quality of evidence was low in respiratory and longevity outcome groups to very low in all others.
Conclusions
Increasing heat exposure was associated with a multitude of detrimental outcomes across diverse body systems. The biological pathways involved are yet to be elucidated, but could include epigenetic and developmental perturbations, through interactions with the placenta and inflammation. This highlights the need for further research into the long-term effects of heat exposure, biological pathways, and possible adaptation strategies in studies, particularly in neglected regions. Heat exposure in-utero has the potential to compound existing health and social inequalities. Poor study design of the included studies constrains the conclusions of this review, with heterogenous exposure measures and outcomes rendering comparisons across contexts/studies difficult.
Professor Matthew Chersich – University of the Witwatersrand; Darshnika Lakhoo – Research Clinician, University of the Witwatersrand; Nicholas Brink – Clinical Researcher, University of the Witwatersrand.
See more from MedicalBrief archives:
Health Department weather alert app warns pregnant women
Extreme heat exacerbating global health risks — UN scientific report
Heat in sub-Saharan Africa tied to miscarriage risk – UKZN study