Heat waves linked to increased rates of preterm and early-term births


A recent study reveals that heat waves significantly elevate the rates of preterm and early-term births, highlighting a critical public health issue.

Heat waves linked to increased rates of preterm and early-term births | Image Credit: © Günter Albers - © Günter Albers - stock.adobe.com.

Heat waves linked to increased rates of preterm and early-term births | Image Credit: © Günter Albers - © Günter Albers - stock.adobe.com.

Heat waves are associated with increased rates of preterm and early-term birth rates, according to a recent study in JAMA Network Open.1


  1. Heat waves are associated with higher rates of preterm and early-term births, posing significant public health concerns.
  2. The study analyzed data from over 53 million births in the 50 largest US metropolitan areas, providing robust evidence for the findings.
  3. Increased risks were notably higher among minority subgroups, especially Black mothers, indicating disparities in heat wave impacts.
  4. The strongest associations were observed with heat waves occurring in the 4 days before birth, suggesting a critical exposure window.
  5. The frequency of heat waves has increased over time, from an average of 1.8 days per year (1993-2004) to 2.4 days per year (2005-2017), potentially exacerbating the risks for preterm and early-term births.

Preterm birth is a leading cause of infant morbidity and mortality, and similar adverse outcomes have been reported following early-term birth. The prevalence of preterm and early-term birth in the United States is 10.5% and 28.8%, respectively, indicating significant public health implications from factors that increase these outcomes.

Data has indicated a potential association between high outdoor temperature in the week before birth and preterm birth. However, it is difficult to evaluate heat waves because of the need for large sample sizes.

A 2020 review found a link between heat exposure and birth outcomes such as preterm birth in 9 of 10 evaluated studies.2 Additionally, increased risks were reported among minority subgroups, especially Black mothers. Investigators recommended additional research to further comprehend risks.

To evaluate the impact of heat waves on preterm and early-term birth rates, investigators conducted a study using birth record data from the 50 largest US metropolitan areas.1 The National Vital Statistics System at the National Center for Health Statistics was assessed for natality data from 1993 to 2017.

Births from mothers living in the 50 largest metropolitan statistical areas (MSAs) were identified using standards codes. Data obtained from birth files included date of birth, state, county, gestational age, maternal age, maternal education level, maternal race and ethnicity, infant sex, parity, induction of labor, and plurality.

The product Daymet was used to obtained daily minimum and maximum temperatures at a 1 × 1-km resolution. Mean daily temperatures were averaged over county-level grids, with estimates combined into a single population-weighted spatial mean for each MSA daily. The US Census population weights were used to perform county population weighting.

Hot days were considered those above the 97.5th percentile for the MSA over the 25-year study period. Heat waves were evaluated as hot day clusters within the 7 days and 4 days before the birthdate.

There were 3 heat wave definitions created: heat wave definition 1 (HW1), heat wave definition 2 (HW2), and heat wave definition 3 (HW3). HW1 was the number of hot days in the 4-day or 7-day period, HW2 consecutive hot days in the period, and HW3 mean degrees Celsius over the threshold during the period.

There were 53,154,816 births included in the final analysis, 2,153,609 of which were preterm births and 5,795,313 early-term births occurring during the warm season. Thirty percent of mothers were aged under 25 years, while 53.8% were aged 25 to 34 years and 16.3% 32 years and older.

A mean 2 days per year per MSA met the definition for a 4-consecutive day heat wave in a 4-day window, and a mean 4.2 days per year per MSA met the definition in a 7-day window. An increase in heat waves was observed over time, from a mean 1.8 days per year per MSA between 1993 and 2004 to a mean 2.4 days per year per MSA between 2005 and 2017.

The 7-day window for HW1 and HW2 had more days indicating heat waves than the 4-day window because of the longer period. Daily rates of both preterm birth and early-term birth were increased by heat waves. More days of heat, more consecutive days of heat, and higher mean degrees over the threshold all increased the association.

A rate ratio of 1.02 was reported for preterm birth and 1.01 for early-term birth following 4 consecutive days of temperatures above the 97.5th percentile. Both rates increased by 1% with every 1 °C increase in mean temperature above the threshold.

These results indicated an association of preterm and early-term births with heat waves in the week before birth. Notably, the strongest associations were reported from heat waves in the 4 days before birth.


  1. Darrow LA, Huang M, Warren JL, et al. Preterm and early-term delivery after heat waves in 50 US metropolitan areas. JAMA Netw Open. 2024;7(5):e2412055. doi:10.1001/jamanetworkopen.2024.12055
  2. Bekkar B, Pacheco S, Basu R, DeNicola N. Association of air pollution and heat exposure with preterm birth, low birth weight, and stillbirth in the US: A systematic review. JAMA Netw Open.2020;3(6):e208243. doi:10.1001/jamanetworkopen.2020.8243 
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