From 1969 to 1988, hot weather cost American children about 151 thousand days of fetal development per year. This conclusion was made by American scientists by comparing the average birth rate in the country at different temperatures. According to their estimates, about 822 children were born prematurely on every day of the heat, and in all, due to such weather, 25 thousand children were born prematurely.

Preterm birth is often associated with a risk to the health of the child: the consequences can be not only for physiology but also for his mental abilities. One of the factors that can cause an early birth of a child is considered hot weather, and scientists have repeatedly tried to find out how significantly the weather affects fertility. 

In early works, for example, scientists estimated the number of premature babies that were born in different weather conditions. Such a technique, however, does not allow us to track the direct effect of temperature: for example, if the pregnancy lasts longer than the prescribed period, then the child who was born as a result of accelerated childbirth provoked by heat will not fall into these statistics.

Alan Barreca from the University of California-Los Angeles and Jessamyn Schaller of Claremont McKenna College have developed a way to gauge how hot weather is pushing the date of delivery. They calculated the average birth rate in the country per day and compared it with the temperature in a given period.

Researchers worked with U.S. data from 1969 to 1988. During this time, they counted about 56 million births in women aged 15-44. To assess the impact of hot weather, they compared the average birth rate for each day of the year with other years when the air was cooler. They also compared fertility with averages during the previous month before the temperature change.

Scientists found that at temperatures above 32.2 degrees, the number of births increased by 0.97 per 100,000 women, compared to a temperature of 15.5 to 21 degrees. This is an increase of about five per cent, with an average of 19 births per day during the study period. In general, the dependence of the number of births on the temperature was uneven: the schedule begins hollow and increases sharply – after about 20 degrees.

The authors then calculated how a single jump in temperature affects fertility. They calculated that the effect of hot weather persists for another day, and on the second day the increase in childbirth is 0.66, after which the birth rate falls back.

Finally, the researchers calculated the overall effect of the hot weather. It turned out that every hot day in the year took an average of 9.9 days of pregnancy per 100 thousand women. Multiplying this by the number of women aged 15–44 in the United States during the study period, they concluded that each additional day of heat cost them five thousand days of pregnancy. Considering that on average in the studied years in the USA there were 30.3 hot days a year, they received 151 thousand lost days of intrauterine development for children born in each of these years.

It is difficult to say exactly how many children could be born prematurely during this period because all estimates are averaged. Scientists tried to get an approximate figure: for this, they divided the total number of days lost by the increase in fertility and received 1.63 children per 100 thousand women 15-44 years old, that is, at least 822 children on each hot day: for 20 years of observation, this is about 25 thousand children.

How exactly the hot weather affects the body of a pregnant woman, however, is still unclear: it is possible that in the heat the release of oxytocin (which also stimulates childbirth) increases or the case is vasospasm that provokes contractions. The authors of the work remind that, according to many climatic forecasts, the number of hot days in the coming years will increase, therefore, premature birth can affect even more children. They believe that air conditioning could solve this problem. However, since the use of air conditioners alone increases greenhouse gas emissions, it may make sense to look for another solution.