Humid heat may increase the risk of premature birth. But aspirin could help

Pregnancy can be a time of joy and anticipation. But it can also be a nerve-wracking experience, with many factors affecting when and how a baby arrives.

A new study, published today, suggests when pregnant women are exposed to high levels of humid heat during pregnancy, they are more likely to have a preterm birth.

However, this study also found taking aspirin at low doses during pregnancy could help reduce this risk. But pregnant women should speak to a doctor before taking aspirin or other medications.

What is a preterm birth?

Preterm birth is when a baby is born prematurely, before 37 weeks of pregnancy. Globally, roughly 10% of babies – or about 13 million infants – are born preterm each year.

Tragically, about one million of these babies do not survive. That makes preterm birth the leading cause of death in children under five.

There are three different types of preterm births:

• extremely preterm, referring to a live birth before 28 weeks
• very preterm, when a baby is born between 28 and 32 weeks
• moderate to late preterm, meaning delivery between 32 and 37 weeks.

Read more: 20% of pregnant Australian women don’t receive the recommended mental health screening

What causes it?

It’s unclear what exactly causes preterm birth. And many cases happen spontaneously, meaning there are no signs a baby will be born early.

However, certain factors may increase a woman’s risk of giving birth prematurely. These include genetics, various infections and chronic conditions such as diabetes and high blood pressure. These risk factors all cause inflammation in the body, which current evidence suggests significantly increases preterm birth risk.

Pregnant women who are exposed to environmental pollutants – such as bushfire smoke and pesticides – may also be more likely to give birth prematurely. This is because these pollutants can contribute to inflammation.

Read more: Pregnant women should take extra care to minimise their exposure to bushfire smoke

The effect of humidity

A growing body of evidence suggests exposure to extreme heat may be another environmental factor that increases preterm birth risk.

Extreme heat can increase levels of specific proteins – known as shock proteins – in the blood of pregnant women. These proteins can trigger inflammation by activating the body’s immune response.

High temperatures may also reduce blood flow to the placenta, limiting the oxygen and nutrients the baby receives.

Humidity adds to this risk. When the air is humid, sweat doesn’t evaporate as easily, making it harder for the body to cool down. This can place extra strain on pregnant women and has been linked to a higher risk of preterm birth.

This may help to explain the high rates of preterm birth in regions that are also most affected by climate change, such as South Asia and sub-Saharan Africa. In these places, where temperatures are high and heatwaves are common, even small increases in heat can impact the health of mothers and newborns.

Read more: Extreme heat can be risky during pregnancy. How to look after yourself and your baby

What this new study involved

A newly published study examined how humid heat exposure during pregnancy affects the risk of preterm birth. It also investigated whether low-dose aspirin might help reduce this risk, possibly because aspirin can improve blood flow and reduce inflammation.

This research was carried out across several countries with hot climates, including the Democratic Republic of Congo, Zambia, Kenya, Guatemala, Pakistan and parts of India.

More than 11,500 pregnant women participated in this trial. About half of them were given a low daily dose of aspirin (81mg) from when they joined the study through to when they were 36 weeks pregnant. The other half received a placebo – a pill with no active ingredients – over the same period. The researchers then compared the birth outcomes of the two groups, and came up with three main findings.

1. Overall, the rate of preterm birth was lower in women who took low-dose aspirin (11.6%) compared with those who took a placebo (13.1%).

2. Among women who were not taking aspirin, each 1°C increase in temperature translated to a noticeable increase (5%) in the risk of preterm birth. This pattern was not seen in women taking low-dose aspirin.

3. Exposure to more heat later in pregnancy was linked to a greater chance of preterm birth in the placebo group, but not in the low-dose aspirin group.

Read more: More and more women in Australia are having their labour induced. Does it matter?

Limitations of this study

This study has two main limitations.

First, it generalised data about temperatures in different cities that may not fully reflect what each woman experienced day-to-day – for example, if their house was hotter or cooler than average. It may also underestimate the length and/or severity of heatwaves. This is because scientists measure temperature in various ways, and may not have access to accurate data from certain locations.

Second, the researchers were not able to determine the exact reasons why some women gave birth early, or whether these differed between the low-dose aspirin and placebo groups.

Overall, this study adds to growing evidence that high temperatures and humidity may increase the risk of preterm birth. It also suggests low-dose aspirin, taken early in pregnancy, may help reduce the risk of heat-related preterm birth.

However, more and larger studies are needed to replicate these findings. And if you’re a pregnant woman who is concerned about preterm birth risk, visit your doctor before taking any aspirin or other medications.

Where to next?

Unfortunately, heatwaves will only become more frequent and intense. So future work should focus on identifying which population groups are most at risk, and how heat affects different stages of pregnancy. Researchers must also test other simple, low-cost strategies that could protect pregnant women from the effects of heat.