Association of Daytime-Only, Nighttime-Only, and Compound Heat Waves With Preterm Birth by Urban-Rural Area and Regional Socioeconomic Status in China

Abstract

ImportanceAssociations between heat waves and preterm birth (PTB) have been reported. However, associations of daytime-only, nighttime-only, and compound heat waves with PTB have yet to be explored at a national level. Furthermore, possible heterogeneity across urban-rural communities with different socioeconomic statuses needs to be explored.ObjectiveTo examine the association between daytime-only, nighttime-only, and compound heat waves and PTB in China and to find variations between urban and rural regions.Design, Setting, and ParticipantsThis case-crossover study used nationwide representative birth data between January 1, 2012, and December 31, 2019, from China’s National Maternal Near Miss Surveillance System. This multisite study covered 30 provinces in China and ensured the representation of urban and rural populations across 3 socioeconomic regions. Singleton live births delivered in the warm seasons from April to October during the study period were included. Exclusion criteria consisted of gestational age younger than 20 or older than 45 weeks, maternal ages younger than 13 or older than 50 years, conception dates earlier than 20 weeks before January 1, 2012, and later than 45 weeks before December 31, 2019, and an inconsistent combination of birthweight and gestational age according to growth standard curves of Chinese newborns. Data were analyzed from September 10, 2021, to April 25, 2023.ExposuresEighteen definitions of heat waves by 3 distinct types, including daytime only (only daily maximum temperature exceeds thresholds), nighttime only (only daily minimum temperature exceeds thresholds), and compound (both daily maximum and minimum temperature exceeds thresholds) heat waves, and 6 indexes, including 75th percentile of daily temperature thresholds for 2 or more (75th-D2), 3 or more (75th-D3), or 4 or more (75th-D4) consecutive days and 90th percentile of daily temperature thresholds for 2 or more (90th-D2), 3 or more (90th-D3), and 4 or more (90th-D4) consecutive days.Main Outcomes and MeasuresPreterm births with less than 37 completed weeks of gestation.ResultsAmong the 5 446 088 singleton births in the final analytic sample (maternal mean [SD] age, 28.8 [4.8] years), 310 384 were PTBs (maternal mean [SD] age, 29.5 [5.5] years). Compared with unexposed women, exposure of pregnant women to compound heat waves in the last week before delivery was associated with higher risk for PTB, with the adjusted odds ratios (AORs) ranging from 1.02 (95% CI, 1.00-1.03) to 1.04 (95% CI, 1.01-1.07) in 6 indexes. For daytime-only heat wave exposures, AORs ranged from 1.03 (95% CI, 1.01-1.05) to 1.04 (95% CI, 1.01-1.08) in the 75th-D4, 90th-D2, 90th-D3, and 90th-D4 indexes. Such associations varied by rural (AOR range, 1.05 [95% CI, 1.01-1.09] to 1.09 [95% CI, 1.04-1.14]) and urban (AOR range, 1.00 [95% CI, 0.98-1.02] to 1.01 [95% CI, 0.99-1.04]) regions during exposure to daytime-only heat waves in the 75th-D3 and 90th-D3 indexes.Conclusions and RelevanceIn this case-crossover study, exposure to compound and daytime-only heat waves in the last week before delivery were associated with PTB, particularly for pregnant women in rural regions exposed to daytime-only heat waves. These findings suggest that tailored urban-rural preventive measures may improve maternal health in the context of climate change.

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

“…The magnitude of associations that we observed for preterm birth and heat waves was generally lower than in previous studies, although these studies show considerable heterogeneity . A 2020 six-study meta-analysis yielded a summary odds ratio for preterm birth after heat waves of 1.16 (95% CI, 1.10-1.23) based on populations from Montreal, Quebec, Canada; Alabama; Brisbane, Australia; Rome, Italy; Sabzevar, Iran; and China .…”

“…The magnitude of associations that we observed for preterm birth and heat waves was generally lower than in previous studies, although these studies show considerable heterogeneity. 8 , 11 , 12 , 13 , 14 , 15 , 16 , 31 A 2020 six-study meta-analysis yielded a summary odds ratio for preterm birth after heat waves of 1.16 (95% CI, 1.10-1.23) based on populations from Montreal, Quebec, Canada; Alabama; Brisbane, Australia; Rome, Italy; Sabzevar, Iran; and China. 7 Previous US national analyses have not had access to the birthdate field 32 or were limited to the 1980s when birthdate and geography were not yet restricted by the NCHS.…”

“…Studying heat waves is considerably more difficult because of the large sample sizes required to estimate modest effects of rare multiday heat events. With a few exceptions, existing studies generally support a positive association between heat waves and preterm birth, but with considerable heterogeneity in the magnitude of effect estimates observed, the definition of heat waves, and the heat thresholds . Although few studies directly address physiological mechanisms, there are several biologically plausible pathways.…”

“…With a few exceptions, existing studies generally support a positive association between heat waves and preterm birth, but with considerable heterogeneity in the magnitude of effect estimates observed, the definition of heat waves, and the heat thresholds. 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 Although few studies directly address physiological mechanisms, there are several biologically plausible pathways. Heat stress and dehydration reduce uterine and placental blood flow, possibly affecting uterine contractility and/or hormone levels governing the induction of labor.…”

Disparities of Heatwave-Related Preterm Birth in Climate Types — China, 2012–2019

“…The data included information from 438 health facilities in 325 counties or districts across China. We applied four exclusion criteria and extracted a final analytic sample of 5,446,088 births in the warm season (April to October), as described elsewhere ( 7 ). The NMNMSS was approved by the Ethics Committee of West China Second University Hospital, Sichuan University, China (Protocol ID: 2012008), and adhered to the principles of the Declaration of Helsinki.…”