By Matthew Chersich, Darshnika Pemi Lakhoo, et al
Climate change is one of the
greatest public health threats humanity has ever faced.
Global warming is part of this
threat. Increasing temperatures are linked to deteriorating health, especially
in vulnerable populations, including pregnant women and children.
Scientists have previously
shown that heat exposure increases the risk of preterm birth and stillbirth.
New research continues to uncover worrying links to poor outcomes for mothers
and their babies. Congenital abnormalities, hypertension in pregnancy and low
birth weight are 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. To explore this question, we conducted a systematic
review of all the existing research on the effects of heat exposure in
pregnancy on health and socioeconomic consequences in later life.
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 they were born
suffered alarming lifelong effects.
Long-term 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 an
individual 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 which 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 US$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 including 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 shown the month a baby is born
has been long associated with the risk of 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 likely 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.
It is our duty to speak out for
those with no voice, who played no part in causing this public health emergency
and who are likely to experience the worst consequences of our inaction.
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