Pregnancy and extreme heat: Reprogramming the placenta

There is a key gap in molecular evidence, particularly in humans, on how heat exposure reshapes the placenta, according to a new review for The Biochemist.

The study, carried out by Foundation-funded Dr Khondoker Akram and Professor Dilly Anumba from the University of Sheffield, emphasises the pivotal role of the placental in sustaining healthy gestation, and the imperative need to advance molecular research in this field.

What are the risks of extreme heat to pregnant women?

The changing climate has already driven global temperatures approximately 1.2°C above pre-industrial levels. Following COP30 - the annual UN climate summit held this year in Belém, Brazil - the Belém Health Action Plan (BHAP) emerged as a major strategic framework.

Designed to mobilise the global community toward building climate-resilient health systems, the plan highlights the need to mitigate future warming, where the continued rise of daily temperatures will inevitably lead to more deaths and other adverse outcomes.

For already vulnerable populations, such as pregnant women, frequent and intense heatwaves pose critical health risks. Studies suggest that infant mortality could rise by 22.4% for every 1°C increase in daily temperature.

Janet, a mother of five in Kenya, recently shared her story with us.

“The heat has hugely affected me and my child during my pregnancy, delivery and after birth."

Pre-term birth is among the most pressing concerns facing maternal and infant health. Defined as birth before 37 weeks of gestation, it is the leading cause of death in children under five worldwide. Studies estimate that the risk of pre-term birth rises by 5% for every 1°C increase in temperature – and by up to 16% during heatwaves.

In low- and middle-income countries, these effects are particularly concerning. Rates of preterm birth are almost twice as high in LMICs than high-income countries, ranging from 10.4% in Asia to 12% in sub-Saharah Africa.

Dr Akram explains:

“Extreme heat is particularly harmful to pregnant women because of the anatomical and physiological changes that occur to support foetal growth.

The foetus typically has a core temperature around 0.5°C higher than the mother’s, due to its own metabolism. This extra heat must be released through the mother’s skin.

However, when the surrounding environment is hot, the body’s ability to dissipate heat becomes impaired, leading to heat accumulation within the womb. This excess heat can cause placental dysfunction and foetal stress.”

Taking a closer look at the placenta

We know that the placenta plays a vital role in sustaining a healthy pregnancy – but what is much lesser understood is the impact of extreme heat on the placenta.

Some insight into the effect of heatwaves on the placenta can be gained from animal studies. For instance, research into dairy cows has shown that exposure to high temperatures during late pregnancy can alter gene expression and epigenetic patterns in the placenta. These changes can affect the placenta’s ability to deliver oxygen and nutrients to the foetus, which in turn can increase inflammation and result in lower birth weight.

To address the need for more research into human placenta, Dr Akram and his team at Sheffield University analysed biological data collected from a small number of placental samples from pregnant women in the UK.

“We found a set of genes that were more active in placentas from pre-term births occurring during warmer months than in those from colder months. These genes are involved in biological pathways that promote inflammation and trigger early labour.”

The team are now conducting a larger study in collaboration with a Nigerian research group, led by Dr Isah Aliyu at Bayero University, that involves placental samples from women in both the UK and Nigeria. With this new direction, they aim to explore how extreme heat affects the placenta at the genetic, epigenetic, and protein levels.

Asides from transporting nutrients, the placenta also functions as an important endocrine gland, releasing hormones that are crucial for maintaining pregnancy and supporting foetal growth. In terms of how extreme heat affects the hormonal function of the placenta, much is yet to be uncovered here too.

Focusing on this knowledge gap, Dr Akram and his team are also investigating how changes in gene expression of human placenta can affect the activation of hormone-like substances, that in turn trigger labour. He aims to investigate how heat can disrupt placental hormonal regulation, and how this can additionally contribute to an increasing prevalence of pre-term births.

What can we do to help mitigate these risks?

Governments and policymakers around the world can implement a number of public health, social care, and infrastructural changes to make a meaningful difference to maternal and infant health.

“Policymakers can help to raise awareness among pregnant women about the risks of heat exposure, early warning signs of heat stress, and strategies for staying cool. This could be introduced during antenatal check-ups. Workplace legislation could also help to regulate extreme heat exposure limits for pregnant women, especially those who work outdoors,” says Dr Akram.

Through advanced molecular research into the impact of environmental heat on the placenta, we can hope to see promising new targeted therapies and evidence-based health policies that better safeguard maternal and infant health.

Read the full review in The Biochemist here >>