Pre-term birth—delivery before 37 weeks of pregnancy—is a leading cause of neonatal death. Complications arising from it are thought to cause 40% of all deaths in babies less than a month old. Each year more than 4.5m women and babies around the world die during pregnancy, childbirth or in the weeks following, about one death every seven seconds.
The relationship between rising temperatures and pre-term births is now well established. A 2024 meta-analysis of 198 studies across 66 countries found that the odds of pre-term birth increase by about 4% for every 1°C rise in the average temperature women are exposed to in the month before giving birth, and more over longer periods. Heatwaves increase the likelihood of pre-term birth by more than a quarter. Strong links have also been found between hotter weather and stillbirths, as well as gestational diabetes and hypertensive disorders in expectant mothers.
Most studies have been conducted in rich, temperate countries, so the true impact could be much larger. In The Gambia, research by Dr Ana Bonell of the London School of Hygiene and Tropical Medicine found that pregnant women working in the fields were exposed to dangerous heat on most days and that in a third of tests their fetuses showed signs of distress. The first large-scale study (2010, California) found that the risk of pre-term birth increased more than twice as much among black mothers than white ones for the same rise in temperature.
In Africa, climate change is estimated to have doubled the number of heat-related deaths in children under five by 2009 compared with the level expected without global warming. On the world's current emissions trajectory, child mortality would rise by 20% in South Africa between 2040 and 2060, and pre-term births would increase by more than half in both South Africa and Kenya.
Scientists estimated that heatwaves caused an average of 13,262 pre-term births each year in China between 2010 and 2020, about a quarter of which were attributable to climate change. In India, about 47,000 heat-related neonatal deaths in the two decades after 2001 were caused by the changing climate; in Nigeria the figure was 31,700.
The WHO lists pre-term birth as a leading cause of lost human capital worldwide at all ages. Babies born too early are far more likely to suffer cardiovascular disease, respiratory issues and developmental disorders throughout their lives. By one estimate, each case in America has a lifetime cost to the economy of $64,815. Heat-related pre-term births in China cost the country more than $1bn a year.
The world's largest study trying to understand why heat affects pregnancy outcomes is led by Dr Jai Das at the Aga Khan University in Karachi, Pakistan. It is recruiting some 6,400 pregnant women and monitoring the exact conditions they face throughout pregnancy, including wearing devices logging temperature and humidity around the clock. The study is supported by the Wellcome Trust, which has awarded £17m to nine projects around the world investigating mothers' and babies' biological vulnerability to extreme heat.
Hypotheses under investigation include altered blood flow to the placenta, hormonal changes and disruption of how fetal cells express certain genes. A separate project, also led by Dr Das, tests cheap interventions in Pakistani homes such as adding canvas shading to roofs or painting walls a reflective colour. High Horizons, a research consortium backed by British and European public money, runs projects including an app offering tailored advice to pregnant women being trialled in Sweden, South Africa and Zimbabwe.
Climate Central, a research group, estimates that climate change has at least doubled the annual number of hot days that can increase pregnancy risks in 222 out of 247 countries. In America, several states now let Medicaid contribute to the cost of air-conditioning for the most vulnerable, though being poor and pregnant is not in itself enough to qualify.
Practically no countries mention maternal health in the climate-adaptation plans they submit to the UN. Only a fraction of the heat-health action plans requested by the WHO address it.
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