METHODOLOGY-How we analyzed the relationship between heat storms and preterm births in Mexico

Over the last decade and a half, medical researchers have studied how ​heat affects pregnancies, and one of the strongest links they’ve found is between heat and preterm births. ‌But ​do heat storms and heat waves correlate with preterm births? We wanted to see for ourselves – and when we looked for a good country to examine, Mexico leapt out. Mexico’s Health Ministry publishes a record of almost every birth in the country, more than 34 million from 2008 through 2025, including the baby’s date of birth, how many weeks into the pregnancy the baby was born, the mother’s age and the municipality where she lives. ‌Mexico also stood out because its population is large – about 130 million people – and the country has remarkably diverse climates.

Here’s how we carried out our analysis. First, Reuters used ERA5 temperature data from the European Center for Medium-Range Weather Forecasts to identify heat waves and heat storms: stretches of at least five days when temperatures stayed in the warmest 10% of the historical range for that place and date. We define a heat storm as any heat wave that at its peak covers at least 1% of the planet, about half the size of Canada.

Using digital maps of Mexico’s municipalities, Reuters measured how many days each ‌mother’s municipality was covered by heat storms and heat waves in the four weeks before each birth. Reuters also determined heat intensity by measuring how far temperatures rose above the historical norm. Combining these two measures – number of days and heat intensity – we calculated an exposure level: none, ‌mild, moderate, heavy, severe or extreme. Because the hottest time of year varies across Mexico, Reuters looked only at births during each municipality’s three hottest months, based on temperatures from 1961 to 1990. For most of the country, that is March through May or April through June, though for some parts of the country it is June through August or July through September. In total, during the hottest seasons from 2008 through 2025, the data show 8.3 million births.

Nationally, 7.2% of babies were born preterm (before 37 weeks) when the mother had no exposure to a heat wave or heat storm in the last four weeks of pregnancy, compared with 8.5% for mothers with the most extreme exposure, which represents an 18% rise in the rate. Reuters ⁠found that the ​preterm birth rate rose with each exposure level, from mild to extreme. Using standard statistical ⁠tests, we determined that each finding is statistically significant, meaning it was unlikely to occur by chance. We repeated the analysis while accounting for the year of birth and the mother’s age. In both cases, we found that the rate of preterm births rose with women’s exposure to heat storms and heat waves in the four weeks before birth. Accounting for the age ⁠of the mother, we found that exposure to heat waves or heat storms correlated with a 6.3% increase in the preterm birth rate over the base rate for mothers who had no exposure.

Many factors could affect preterm births in a given year. For example, COVID peaked in 2020 and 2021, and the world suffered a recession ​in 2008 and 2009. However, accounting for such one-off factors in a baby’s year of birth entails a complication: Heat storms and heat waves have been increasing over time, making it impossible to fully disentangle the rise in heat from other factors that may bear ⁠on the year of birth. Still, when accounting for the year of birth, the rate of preterm births rose by 4.6% with exposure to heat storms and heat waves. For the analysis of births in the temperate mountain regions of Chiapas state, we measured elevation at 25 random points within each Chiapas municipality and classified the municipality as above 1,000 meters if the median elevation was ⁠higher. ​We classified a municipality as "temperate year-round" if, from 1961 to 1990, its average daily high was between 22 C and 27 C (71.6 F and 80.6 F) in every month, or if its warmest and coolest months differed by less than 6 C (10.8 F) and its yearly average high fell in that range. We ran our analysis on births only in municipalities that met both criteria, finding that the rate of preterm births rose 9.6% for mothers with exposure to heat storms and heat waves over the baseline rate for mothers with no exposure. This and the other findings referred ⁠to in this article were statistically significant.

Finally, here is a more detailed explanation of how we identify and map heat waves and heat storms, and how we calculate their intensity levels. SOURCES

BIRTHS Dirección General de Información en Salud – Subsistema de Información sobre Nacimientos (Directorate General of Health Information – Birth Information Subsystem), 2008–2025.

MUNICIPAL ⁠BOUNDARIES Instituto Nacional de Estadística y Geografía (National Institute of Statistics and Geography)

ELEVATION Shuttle Radar Topography Mission ⁠digital elevation dataset via opentopodata.org.

CLIMATE Copernicus Climate Change Service, Climate Data Store (2023). ERA5 hourly data on single levels from 1940 to present. Copernicus Climate Change Service (C3S) Climate Data Store (CDS). https://doi.org/10.24381/cds.adbb2d47 (Reuters updates the data daily.)

Generated using Copernicus Climate Change Service information. Neither the European Commission nor the European Centre for Medium-Range Weather Forecasts (ECMWF) is responsible for any use that may be made of the Copernicus information or data it contains. For full documentation, click here.