Women who were exposed to a heatwave the week before birth were more likely to have a pre-term delivery finds a new US study, which highlights the potential impact of climate change on the health and wellbeing of mums and their babies.
The research, published in Environment International, also found that the hotter the temperature or the longer the heatwave, the greater the risk. Longer heatwaves were associated with the highest risk of a pre-term birth.
"We looked at acute exposure to extreme heat during the week before birth, to see if it triggered an earlier delivery," said first author Sindana Ilango,"We found a consistent pattern: exposure to extreme heat does increase risk. And, importantly, we found that this was true for several definitions of 'heatwave."
Senior author Tarik Benmarhni said no one had explored the kinds of conditions that could trigger pre-term births. "Is it the temperature? Is it the combination of the temperature and the humidity? Is it the duration of the heatwave?" she explained. "It's important to ask these questions to know when we need to intervene and inform pregnant people to stay inside and stay cool."
To examine this connection, a team from the University of California used data from California Department of Public Health on every single birth in California between 2005 and 2013. The researchers sorted the two million births that occurred during the summer months into postcodes and compared the birth outcome data to environmental records for that area at the time the person went into labor.
Results showed that the rate of pre-term births was around seven per cent of all pregnancies. "Using the most conservative definition of a heatwave, the risk of pre-term birth was 13 per cent higher among mothers who experienced a heatwave during the last week of gestation compared to mothers who did not," the authors write. "These findings suggest that acute exposure to extreme heat during warm-weather months during the final weeks of gestation may trigger earlier deliveries and increase risk of pre-term birth."
Ms Ilango called the strength of the trend "surprising." "It was so clear that as temperature and duration of a heatwave went up, so did the risk of pre-term birth."
But the question is why?
"We think mothers are more susceptible to changes in hormones and thermoregulation toward the end of gestation," Ms Ilango tells Essential Baby. "Perhaps experiencing extreme heat triggers an earlier delivery because of increased dehydration resulting in changes in hormones that promote contractions."
Australian OB/GYN Dr Steve Robson says we've recognised for around a decade or so now that heatwaves, "seem to usher in a wave of pre-term births".
"A study from Brisbane showed that the risk of pre-term birth almost doubles with severe heatwaves," he tells Essential Baby, adding that this is important as we spend a lot of research time looking at factors such as infection as a cause of pre-term birth. "The weather often escapes our notice as a factor because it isn't included in birth statistics," Dr Robson continues. "But when researchers check weather conditions and add that information to mundane birth information, a surprise story emerges -heatwaves definitely increase the chance of pre-term birth in otherwise healthy women."
Dr Robson says it's important information for women to know. "Over the summer we all got a taste of a possible climate future," he says. "If we're facing longer, hotter, summers then we all should be worried for the health of the next generation. Premature birth has all sorts of potential
negative health consequences for babies."
So what can pregnant women do?
"Keep an eye to the weather and avoid being outside when it's really hot," Dr Robson says. "Plenty of fluid, use a fan or air conditioning if its possible. If you are concerned during heatwaves make sure you let your midwife or doctor know as soon as possible."