When it comes to pregnancy and babies, the closer to full-term the better. Compared to full-term pregnancies, we know that near-term babies have a higher risk of medical problems soon after birth, and lower cognitive outcomes later in childhood.
So, with that in mind, we have some bad news. Researchers have found that extreme heat makes babies rush to the exit sooner, leading to an average of 25,000 US infants a year born a little early due to hot weather.
And like nearly everything else in this world, it's only going to get worse with climate change.
"Given recent increases in the frequency of extremely hot weather, there is a clear need to better forecast the potential magnitude of climate change's impact on infant health at the national level," the team explains in their paper.
"We find that extreme heat causes an increase in deliveries on the day of exposure and on the following day and show that the additional births were accelerated by up to two weeks."
The two researchers - economists Alan Barreca from the University of California, Los Angeles and Jessamyn Schaller from Cambridge - used US birth rates and temperature data between 1969 and 1988 to get a handle on just how many extra babies were being born on hot days.
Although past studies have looked at this from the view of gestational length, the researchers explain this can be affected by misreporting and can lead to less accurate results.
Instead, the team looked at daily birth rates across the whole of the US, including a whopping 56 million births in their data.
"We advance the methodology of temperature–gestation studies by using data on daily birth rates, as opposed to recorded gestational lengths," the team explain.
This works by analysing the data for a spike in births on hot days, and then checking if there is a subsequent decrease afterwards.
"For example, in a given county of the United States, an increase in birth rates on the day of hot weather followed by a decrease two days later suggests temperature reduced gestational lengths by two days."
They estimate that in the US, this led to 25,000 babies a year that were born a little earlier than scheduled due to heat exposure, creating a total loss of over 150,000 gestational days annually.
Although a couple of days less gestation time isn't an issue for every baby, shorter gestation times have been linked to lower health and cognitive outcomes, and giving babies the best start in life means we should probably be trying to keep them a little cooler.
We know that our climate is heating up, and without serious action, it's going to get worse. This means more hurricanes, wildfires, and heatwaves.
What this study shows is that it's not just the humans who have already been born that will be feeling the negative effects of the mercury rising.
"At the end of the century (2080–2099), we estimate that there will be approximately 253,000 additional lost days of gestation per year on average in the United States, affecting nearly 42,000 additional births," the team write.
When it comes to our inaction on climate change, looks like we've got one more thing to explain to the next generation.
The paper has been published in Nature Climate Change.