Preterm and Early-Term Delivery After Heat Waves in 50 US Metropolitan Areas

Abstract

Importance: Heat waves are increasing in frequency, intensity, and duration and may be acutely associated with pregnancy outcomes.

Objective: To examine changes in daily rates of preterm and early-term birth after heat waves in a 25-year nationwide study.

Design, setting, and participants: This cohort study of singleton births used birth records from 1993 to 2017 from the 50 most populous US metropolitan statistical areas (MSAs). The study included 53 million births, covering 52.8% of US births over the period. Data were analyzed between October 2022 and March 2023 at the National Center for Health Statistics.

Exposures: Daily temperature data from Daymet at 1-km2 resolution were averaged over each MSA using population weighting. Heat waves were defined in the 4 days (lag, 0-3 days) or 7 days (lag, 0-6 days) preceding birth.

Main outcomes and measures: Daily counts of preterm birth (28 to <37 weeks), early-term birth (37 to <39 weeks), and ongoing pregnancies in each gestational week on each day were enumerated in each MSA. Rate ratios for heat wave metrics were obtained from time-series models restricted to the warm season (May to September) adjusting for MSA, year, day of season, and day of week, and offset by pregnancies at risk.

Results: There were 53 154 816 eligible births in the 50 MSAs from 1993 to 2017; 2 153 609 preterm births and 5 795 313 early-term births occurring in the warm season were analyzed. A total of 30.0% of mothers were younger than 25 years, 53.8% were 25 to 34 years, and 16.3% were 35 years or older. Heat waves were positively associated with daily rates of preterm and early-term births, showing a dose-response association with heat wave duration and temperatures and stronger associations in the more acute 4-day window. After 4 consecutive days of mean temperatures exceeding the local 97.5th percentile, the rate ratio for preterm birth was 1.02 (95% CI, 1.00-1.03), and the rate ratio for early-term birth was 1.01 (95% CI, 1.01-1.02). For the same exposure, among those who were 29 years of age or younger, had a high school education or less, and belonged to a racial or ethnic minority group, the rate ratios were 1.04 (95% CI, 1.02-1.06) for preterm birth and 1.03 (95% CI, 1.02-1.05) for early-term birth. Results were robust to alternative heat wave definitions, excluding medically induced deliveries, and alternative statistical model specifications.

Conclusions and relevance: In this cohort study, preterm and early-term birth rates increased after heat waves, particularly among socioeconomically disadvantaged subgroups. Extreme heat events have implications for perinatal health.

Figure 1.. Metropolitan Statistical Areas Included in the Study

Circles are proportional to sample size.

Figure 2.. Rate Ratios (RRs) and 95% CIs for Heat Waves Occurring in the 4 Days or 7 Days Preceding Birth (97.5th Percentile Threshold)

HW1 indicates heat wave definition 1: total number of hot days in the 4-day (or 7-day) window, represented by an ordinal variable with categories of 0, 1, 2, 3, or 4 days or more; HW2, heat wave definition 2: consecutive hot days in the 4-day (or 7-day) window, represented by binary indicators for 2 or more consecutive days, 3 or more consecutive days, or 4 or more consecutive days; and HW3, heat wave definition 3: mean degrees Celsius over the threshold during the exposure window, a continuous variable calculated as the 4-day (or 7-day) moving mean − the 97.5% threshold, and if less than 0, then set to 0.

Figure 3.. Rate Ratios (RRs) and 95% CIs From Sensitivity Analyses Compared With the Primary Analysis for Heat Waves in the 4-Day Exposure Window

HW2 indicates heat wave definition 2: consecutive hot days in the 4-day (or 7-day) window, represented by binary indicators for 2 or more consecutive days, 3 or more consecutive days, or 4 or more consecutive days; HW3, heat wave definition 3: mean degrees Celsius over the threshold during the exposure window, a continuous variable calculated as the 4-day (or 7-day) moving mean − the 97.5% threshold, and if less than 0, then set to 0 (results for heat wave definition 1 are available in eTable 4 in Supplement 1).

Figure 4.. Subgroup Rate Ratios (RRs) and 95% CIs per 1 °C Increase in Mean Temperature Above the 97.5th Percentile Threshold (HW3) During the 4 Days Preceding Birth

^aIncluding American Indian, Alaska Native, Asian, Other Pacific Islander, and unknown.