24th March 2025
Extreme heat increases complications in pregnancy by about 25%, said Professor Jane Hirst, chair in global women’s health at the George Institute for Global Health, UK, at Imperial College, at a conference on green maternity care at the Royal College of Obstetricians and Gynaecologists.
Hirst was quoting from a systematic review of 198 studies that showed that preterm birth, the commonest cause of death in children under 5 years, is increased by 26% (95% confidence intervals 1.08-1.47), congenital anomalies by 28% (95% confidence intervals 1.09-1.42), gestational diabetes by 28% (95% confidence intervals 1.05-1.74), and hypertensive disorders by16% (95% confidence intervals 1.1-1.22).
Stillbirths also seemed to increase by 13%, although the confidence intervals include 1 (95% confidence intervals 0.95-1.34).
Most of the studies have been of preterm birth and low birthweight, and the effect of extreme heat on many obstetric complications has been hardly studied. In addition, most of the studies come from high-income countries that experience extreme heat (US, Australia). There don’t seem to be any studies from the UK, although London has already seen a temperature of over 40C and, with climate change, heatwaves will become more extreme and last longer. There is a need for more studies of unstudied complications and of studies from low and middle income countries.
Hirst described a prospective study of extreme heat on 800 pregnant women in workplaces in Tamil Nadu which was led by Shanmugam Rekha and in which she had participated. Almost half of the women (378, 47.3%), particularly those working in the fields and brick factories, were exposed to extreme heat, which was defined as heat exceeding a wet-bulb globe temperature of 27.5°C for a heavy workload and 28.0°C for a moderate workload. (Hirst pointed out that these definitions were derived from men in the American army in the 1960s.) The women exposed to extreme heat were similar in most ways to those not exposed. The women exposed to extreme heat compared with those not exposed had a more than doubled chance of a combined measure of miscarriage, low birth weight, preterm birth, and still birth (odds ratio 2.2 95% confidence intervals 1.4-3.3) and a tripled chance of miscarriage alone (odds ratio 3.1 95% confidence intervals 1.3-7.3).
Summing up the study, Hirst said that extreme heat could cause eight deaths in a hundred births.
A group from South Africa led by Mathew Chersich has developed a framework of how to think about interventions to reduce the harm to pregnant women from extreme heat which is shown in the figure below.
https://pmc.ncbi.nlm.nih.gov/articles/PMC10087975/
Behavioural interventions include “maintaining hydration during hot periods, accessing cool areas, or seeking shade; using low‐cost water sprays; wearing appropriate clothing, ideally cotton; and self‐monitoring for symptoms of heat illness, especially dizziness, heavy sweating, fatigue, and clammy skin with goosebumps.” Avoiding heavy work during extreme heat is an obvious response, but it may be impossible for poor women, although Hirst described a micro-insurance scheme that would pay pregnant women when they missed work.
Early warning systems are central to health system responses, but other changes might include holding antenatal clinics early in the morning.
Air conditioning might be the first building change that comes to mind, and it has been described as “tantamount to a potentially life‐saving medical device”; but it’s also been called “a clear example of maladaptation to climate change.” More useful in low-resource settings might be awnings, overhangs, louvres, and insulated walls and roofs. White reflective paint for rooftops can reflect almost all solar radiation. Nature-based interventions may include street trees, green roofs, green walls, ponds and other water features.
A wide range of policy, structural, and financial interventions are possible, and Hirst pointed people to the recent guidance from the UK Health Security Agency on Hot Weather Risks and Their Impact on Health. But the guidance makes no reference to pregnant women, who are clearly a vulnerable group.