Ambient temperature and the risk of preterm birth: A national birth cohort study in the mainland China

Impacts of heat exposure on pregnant women, fetuses and newborns: a systematic review and meta-analysis

Climate Change has wide-ranging and severe health impacts, especially for vulnerable groups. We systematically reviewed the literature (n=198 studies) on heat impacts on maternal, fetal, and neonatal health, conducted meta-analyses to quantify impacts, analysed periods of susceptibility, and graded certainty. Studies covered 66 countries and 23 outcomes. Our results showed increased odds of preterm birth of 1.04 (95%CI=1.03, 1.06) per 1°C increase in heat exposure and 1.26 (95%CI=1.08, 1.47) during heatwaves. Similar patterns were shown for stillbirths and congenital anomalies. Gestational diabetes mellitus odds increased by 28% (95%CI=1.05, 1.74) at higher exposures, whileodds of any obstetric complication increased by 25% (95%CI=1.09, 1.42) during heatwaves. Patterns in susceptibility windows vary by condition. The review demonstrated that escalating temperatures pose major threats to maternal and child health globally. Findings could inform research priorities and selection of heat-health indicators. Clearly more intensive action is needed to protect these vulnerable groups.

Heat exposure induced risks of preterm birth mediated by maternal hypertension

Heat exposure is associated with an increased risk of preterm birth (PTB), with previous work suggesting that maternal blood pressure may play a role in these associations. Here we conducted a cohort study of 197,080 singleton live births across 8 provinces in China from 2015 to 2018. The study first estimated the associations between heat exposure, maternal hypertension and clinical subtypes of PTB, and then quantified the role of maternal hypertension in heat and PTB using mediation analyses. We show that heat exposure (>85th, 90th and 95th percentiles of local temperature distributions) spanning from conception to the 20th gestational week was associated with a 15-21% increase in PTB, and a 20-22% increase in medically indicated PTB. Heat exposure is likely to increase the risk of maternal hypertension and elevated blood pressure. Maternal hypertension mediated 15.7% and 33.9% of the effects of heat exposure (>90th percentile) on PTB and medically indicated PTB, respectively. Based on this large-population study, we found that exposure to heat in early pregnancy can increase the risk of maternal hypertension, thereby affecting the incidence of PTB.

Increased risk of preterm birth due to heat exposure during pregnancy: Exploring the mechanism of fetal physiology

BACKGROUND

Heat exposure during pregnancy can increase the risk of preterm birth (PTB) through a range of potential mechanisms including pregnancy complications, hormone secretion and infections. However, current research mainly focuses on the effect of heat exposure on pathophysiological pathways of pregnant women, but ignore that maternal heat exposure can also cause physiological changes to the fetus, which will affect the risk of PTB.

OBJECTIVE

In this study, we aimed to explore the mediating role of fetal heart rate (FHR) in the relationship between maternal heat exposure and PTB incidence.

METHODS

We assigned heat exposure to a multi-center birth cohort in China during 2015-2018, which included all 162,407 singleton live births with several times FHR measurements during the second and third trimesters. We examined the associations between heat exposure, FHR and PTB in the entire pregnancy, each trimester and the last gestational month. The inverse odds ratio-weighted approach applied to the Cox regression was used to identify the mediation effect of heat exposure on PTB and its clinical subtypes via FHR.

FINDINGS

Exposure to heat significantly increased the risk of PTB during the third trimester and the entire pregnancy, hazard ratios and 95 % CIs were 1.266 (1.161, 1.379) and 1.328 (1.218, 1.447). Heat exposure during the third trimester and entire pregnancy increased FHR in the third trimester by 0.24 bpm and 0.14 bpm. The proportion of heat exposure mediated by FHR elevation on PTB and its subtype ranged from 3.68 % to 24.06 %, with the significant mediation effect found for both medically indicated PTB and spontaneous PTB.

CONCLUSIONS

This study suggests that heat exposure during pregnancy has an important impact on fetal health, and FHR, as a surrogate marker of fetal physiology, may mediate the increased risk of PTB caused by extreme heat. Monitoring and managing physiological changes in the fetus would constitute a promising avenue to reduce adverse birth outcomes associated with maternal heat exposure.

High ambient temperature may increase the risk of anemia in pregnancy: Identifying susceptible exposure windows

Anemia in pregnancy (AIP) is associated with multiple severe maternal and perinatal adverse outcomes. However, there is a lack of evidence on the association between environmental factors and AIP. Aim to explore the association between ambient temperature and the risk of AIP, and identify susceptible exposure windows, we conducted a matched case-control study from 2013 to 2016 in Xi'an, China, which included 710 women with AIP and 1420 women without AIP. The conditional logistic regression model was used to evaluate the association between ambient temperature and AIP at different gestational weeks and gestational months. The association between extreme temperature and AIP was evaluated using the distributed lag nonlinear model (DLNM). We conducted stratified analyses of age, parity, and season of conception, and estimated the interaction between ambient temperature and air pollutants on AIP. Ambient temperature was significantly positively associated with the risk of AIP, and the susceptible exposure windows were 2-25 gestational weeks and 1-6 gestational months, respectively. The strongest effect was observed in the week 8 and month 2, for each 1 °C increase in weekly and monthly mean temperature, the odds ratio (OR) for AIP was 1.038 (95 % confidence interval (CI): 1.022, 1.055) and 1.040 (95 % CI: 1.020, 1.060), respectively. Extreme heat may increase the risk of AIP. Stratified analyses showed that there was no significant difference among different age, parity, and season of conception groups. No significant interaction effect of ambient temperature with air pollution on AIP was found. In summary, high ambient temperature may increase the risk of AIP, and the first and second trimesters may be susceptible exposure windows. Understanding the effect of temperature on pregnant women will be beneficial to reduce the occurrence of AIP.

Net benefit of smaller human populations to environmental integrity and individual health and wellbeing

Introduction

The global human population is still growing such that our collective enterprise is driving environmental catastrophe. Despite a decline in average population growth rate, we are still experiencing the highest annual increase of global human population size in the history of our species-averaging an additional 84 million people per year since 1990. No review to date has accumulated the available evidence describing the associations between increasing population and environmental decline, nor solutions for mitigating the problems arising.

Methods

We summarize the available evidence of the relationships between human population size and growth and environmental integrity, human prosperity and wellbeing, and climate change. We used PubMed, Google Scholar, and Web of Science to identify all relevant peer-reviewed and gray-literature sources examining the consequences of human population size and growth on the biosphere. We reviewed papers describing and quantifying the risks associated with population growth, especially relating to climate change.

Results

These risks are global in scale, such as greenhouse-gas emissions, climate disruption, pollution, loss of biodiversity, and spread of disease-all potentially catastrophic for human standards of living, health, and general wellbeing. The trends increasing the risks of global population growth are country development, demographics, maternal education, access to family planning, and child and maternal health.

Conclusion

Support for nations still going through a demographic transition is required to ensure progress occurs within planetary boundaries and promotes equity and human rights. Ensuring the wellbeing for all under this aim itself will lower population growth and further promote environmental sustainability.

Independent and interaction effects of prenatal exposure to high AQI and extreme Humidex on the risk of preterm birth: A large sample population study in northern China

The combined effects of air pollution and extreme temperature on PTB remain unclear. To evaluate the independent effect and interaction effect of prenatal extreme exposure to air quality index (AQI) and Humidex, on PTB. Based on the National Health Care Data Platform of Shandong University, women who gave birth in 2019-2020 were selected for the study. First, the independent effects of AQI and Humidex on PTB were assessed by logistic regression model. Subsequently, the interaction effects of AQI and Humidex on PTB were estimated separately by calculation of the relative excess risk of interaction (RERI). A total of 34365 pregnant women were included and 1975 subjects were diagnosed with PTB. We observed a significant increase in the odds of PTB associated with maternal high AQI exposure, with an OR of 1.70 (95% CI: 1.59, 1.81). Similarly, extreme exposure to Humidex also demonstrated an elevated PTB odds, with a low Humidex OR of 2.48 (95% CI: 2.23, 2.76) and a high Humidex OR of 1.48 (95% CI: 1.31, 1.67). Finally, we observed an interaction between high AQI and extreme Humidex during the 1st trimester. Interaction effects were noted between high AQI and low Humidex throughout the entire trimester and the 2nd trimester. This study suggests that prenatal exposure to high AQI and extreme Humidex could increase the odds of PTB, with effects exhibiting the sensitivity window and a cumulative trend. Additionally, there is an interaction between AQI and Humidex.

Spatial-temporal distribution of preterm birth in China, 1990-2020: A systematic review and modelling analysis

BACKGROUND

Little is known about the long-term trends of preterm birth rates in China and their geographic variation by province.

OBJECTIVES

To estimate the annual spatial-temporal distribution of preterm birth rates in China by province from 1990 to 2020.

DATA SOURCES

We searched PubMed, EMBASE, Web of Science, CNKI, WANFANG and VIP from January 1990 to September 2023.

STUDY SELECTION AND DATA EXTRACTION

Studies that provided data on preterm births in China after 1990 were included. Data were extracted following the Guidelines for Accurate and Transparent Health Estimates Reporting.

SYNTHESIS

We assessed the quality of each survey using a 9-point checklist. We estimated the annual preterm birth risk by province using Bayesian multilevel logistic regression models considering potential socioeconomic, environmental, and sanitary predictors.

RESULTS

Based on 634 survey data from 343 included studies, we found a gradual increase in the preterm birth risk in most provinces in China since 1990, with an average annual increase of 0.7% nationally. However, the preterm birth rates in Inner Mongolia, Hubei, and Fujian Province showed a decline, while those in Sichuan were quite stable since 1990. In 2020, the estimates of preterm birth rates ranged from 2.9% (95% Bayesian credible interval [BCI] 2.1, 3.8) in Inner Mongolia to 8.5% (95% BCI 6.6, 10.9) in Jiangxi, with the national estimate of 5.9% (95% BCI 4.3, 8.1). Specifically, some provinces were identified as high-risk provinces for either consistently high preterm birth rates (e.g. Jiangxi) or relatively large increases (e.g. Shanxi) since 1990.

CONCLUSIONS

This study provides annual information on the preterm birth risk in China since 1990 and identifies high-risk provinces to assist in targeted control and intervention for this health issue.

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

What is already known about this topic?

An association between prenatal heatwave exposure and the risk of preterm birth was found. However, the disparities in heatwave-related preterm birth across different climate types have not been examined.

What is added by this report?

This nationwide case-crossover study investigated the association between heatwave exposure and preterm birth across different Köppen-Geiger climate types. Among pregnant women residing in the arid-desert-cold climate type, exposure to compound heatwaves was found to be associated with a significantly higher risk of preterm birth {adjusted odds ratios (AORs) ranged from 1.55 [95% confidence interval ( CI): 1.21-1.97] to 2.11 (95% CI: 1.35-3.31)}. In contrast, among pregnant women residing in the tropical monsoonal climate type, exposure to daytime-only heatwaves was associated with an increased risk of preterm birth [AORs ranged from 1.25 (95% CI: 1.03-1.51) to 1.37 (95% CI: 1.05-1.77)].

What are the implications for public health practice?

Specific interventions should be implemented in China to mitigate the risk of preterm birth related to heatwaves, particularly for pregnant women residing in arid-desert-cold and tropical monsoonal climates.

Disparities in preterm birth following the July 1995 Chicago heat wave

PURPOSE

To evaluate if changes in preterm birth (PTB, <37 weeks of gestation) incidence differed between non-Hispanic (NH) Black and NH white births following the July 1995 Chicago heat wave-among the most severe U.S. heat waves since 1950.

METHODS

We used an ecologic study design. We obtained birth data from January 1990-December 1996 from the National Vital Statistics File to calculate the mean monthly PTB incidence in Chicago's Cook County, Illinois. Births between July 1995 and February 1996 were potentially exposed to the heat wave in utero. We generated time series models for NH Black and NH white births, which incorporated synthetic controls of Cook County based on unexposed counties. We ran a secondary analysis considering socioeconomic status (SES).

RESULTS

From 1990-1996, the mean monthly PTB incidence among NH Black births was 18.6% compared to 7.8% among NH white births. The mean monthly PTB incidence among NH Black births from August 1995-January 1996 was 16.7% higher than expected (three additional PTBs per 100 live births per month [95% confidence interval (CI): 1, 5]). A similar increase occurred among low-SES NH Black births. No increase appeared among NH white births.

CONCLUSIONS

Severe heat waves may increase racial disparities in PTB incidence.

Association between low ambient temperature during pregnancy and adverse birth outcomes: A systematic review and meta-analysis

BACKGROUND

Extreme temperature events, including extreme cold, are becoming more frequent worldwide, which might be harmful to pregnant women and cause adverse birth outcomes. We aimed to investigate the association between exposure to low ambient temperature in pregnant women and adverse birth outcomes, such as preterm birth, low birth weight, and stillbirth, and to summarize the evidence herein.

METHODS

Relevant studies were searched in PubMed, Cochrane, and Embase electronic databases until November 2021. Studies involving low ambient temperature, preterm birth, birth weight, and stillbirth were included. The guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-analyses were followed to conduct this study risk of bias and methods for data synthesis.

RESULTS

A total of 34 studies were included. First, pregnant women exposed to low ambient temperature had an increased risk of preterm birth (risk ratio [RR] 1.08; 95% confidence interval [CI] 1.04-1.13). Subgroup analyses revealed that exposure during late pregnancy was more likely to induce preterm birth. In addition, only pregnant women exposed to <1st percentile of the mean temperature suffered increased risk of preterm birth. Moreover, pregnant women living in medium or hot areas were more prone to have preterm births than those in cold areas when exposed to low ambient temperatures. Asians and Blacks were more susceptible to low ambient temperatures than Caucasians. Second, pregnant women exposed to low ambient temperature had an increased risk of low birth weight (RR 1.07; 95% CI 1.03-1.12). Third, pregnant women had an increased risk of stillbirth while exposed to low ambient temperature during the entire pregnancy (RR 4.63; 95% CI 3.99-5.38).

CONCLUSIONS

Exposure to low ambient temperature during pregnancy increases the risk of adverse birth outcomes. Pregnant women should avoid exposure to extremely low ambient temperature (<1st percentile of the mean temperature), especially in their late pregnancy. This study could provide clues for preventing adverse outcomes from meteorological factors.

REGISTRATION

No. CRD42021259776 at PROSPERO ( https://www.crd.york.ac.uk/PROSPERO/ ).

Maternal Exposure to Trace Elements, Toxic Metals, and Longitudinal Changes in Infancy Anthropometry and Growth Trajectories: A Prospective Cohort Study

Exploration of stage-specific effects of maternal exposure to trace elements and toxic metals on infancy continuous growth and trajectories is critical for early-life health management. Within a Chinese prospective cohort in 2014-2015, a total of 919 mother-infant pairs were included, and the urinary levels of 17 elements including vanadium (V), chromium (Cr), manganese, iron, cobalt, nickel, copper, zinc, arsenic, molybdenum, palladium, cadmium, tin, gold, mercury, thallium, and lead in early (mean: 11.9 weeks), and late pregnancy (mean: 32.4 weeks) were assessed. Standardized anthropometric assessments of infants were conducted at 1, 3, 6, 8, and 12 months of age. A three-step longitudinal and high-dimensional data analysis procedure was carried out to estimate the impacts of exposome on dynamic growth. Early-pregnancy exposures to V and Cr were positively associated with repeated measurements of length-for-age z-scores (LAZ). Six trajectories were identified based on LAZ. Maternal single exposure to V and Cr as well as mixed exposure to trace elements in early pregnancy were associated with raised odds for the high-stable group. Our results suggested positive associations between maternal trace element exposome and infancy dynamic growth. V and Cr were the key elements and the early pregnancy might be the critical window.

Extreme Temperature Exposure and Risks of Preterm Birth Subtypes Based on a Nationwide Survey in China

BACKGROUND

Previous studies have reported that ambient temperature may affect perinatal outcomes. However, whether extreme temperature affects the risk of preterm birth (PTB) remains controversial. Studies on the associations of extreme temperature with PTB subtypes are lacking.

OBJECTIVES

We aimed to investigate the associations of extreme climate events with the risks of PTB and its subtypes, discerning possible modifiers.

METHODS

Data on all singleton deliveries were obtained from the China Labor and Delivery Survey (CLDS), a nationwide investigation implemented in 2015 and 2016. PTB was defined as gestational weeks < 37 and then categorized as early (24-34 wk) and late PTBs (35-36 wk), and clinical subtypes [spontaneous PTB, preterm premature rupture of the fetal membranes (PPROM), iatrogenic PTB]. Ambient temperature data were provided by the China National Weather Data Sharing System. Five heat indexes and five cold indexes were used to define heat waves and cold spells. Generalized linear mixed models with a random term by hospital unit were used to assess the associations of short-term prenatal extreme temperature exposure. The Cox proportional hazard regression model was applied to assess the nonlinear associations of low- or high-temperature exposure at the whole and different trimesters of pregnancy with the risk of PTB. Stratified analyses were conducted to assess the possible modification by geographic region and fetal sex.

RESULTS

A total of 70,818 singleton births from 96 hospitals in China were included, among which 4,965 (7.01%) were PTBs. Exposure to extreme cold events 1 wk before delivery was associated with an increased PTB risk, with an adjusted odds ratio (aOR) [95% confidence intervals (CIs)] of 1.07 (95% CI: 1.04, 1.10) and 1.06 (1.04, 1.09) for the total days when the daily average temperature below the fifth percentile (fifth-days) and the 10th percentile (10th-days), 1.18 (1.04, 1.34) for the cold spells when the daily average temperature below the fifth percentile for two consecutive days (fifth-2D), 1.09 (1.03, 1.16) and 1.12 (1.06, 1.19) for the cold spells when the daily average temperature below the 10th percentile for three and two consecutive days (10th-3D and 10th-2D), respectively. Results of extreme temperature exposure during 2 weeks before delivery showed similarly significant associations. The association between cold spells and PTB tended to be stronger for late PTB than for early PTB. Cold spells were mainly associated with spontaneous PTB and late PPROM. A stratified analysis indicated that pregnant women in western and northern regions tended to be more sensitive to cold spells, and pregnant women with a female fetus appeared to be at a higher risk of PTB when exposed to cold spells. Pregnant women in late pregnancy were more susceptible to extreme temperatures. No significant or stable association was found between heat waves and preterm birth.

DISCUSSION

Exposure to cold spells was associated with an increased risk of PTB, especially late, spontaneous PTB and PPROM. The associations appeared to be more pronounced in the north and west regions and in pregnancies with female fetuses. https://doi.org/10.1289/EHP10831.

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

Importance

Associations 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.

Objective

To 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 Participants

This 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.

Exposures

Eighteen 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 Measures

Preterm births with less than 37 completed weeks of gestation.

Results

Among 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 Relevance

In 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.

Extreme temperature exposure increases the risk of preterm birth in women with abnormal pre-pregnancy body mass index: a cohort study in a southern province of China

Background

Prior literature has found that extreme temperature exposure is associated with preterm birth (PTB). However, current evidence provides heterogeneous conclusions, and data on extreme cold and across different pre-pregnancy body mass index (BMI) statuses are limited.

Methods

We conducted a population-based retrospective cohort of 251,257 women between 2014 and 2017 in Guangdong, China, to evaluate whether the association between extreme temperature exposure and PTB varied in pre-pregnancy BMI status. Participants were divided into three categories based on pre-pregnancy BMI: underweight (BMI < 18.5 kg/m2), normal weight (18.5-23.9 kg/m2), overweight or obesity (≥ 24.0 kg/m2). We fitted Cox proportional hazards models to assess the association between daily mean temperature and PTB at each trimester for each BMI category separately. The hazard ratios (HRs) at the 5th and 95th percentiles of temperature (defined as low and high temperatures respectively) were provided using the median temperature at each trimester as a reference.

Results

58,220 (23.2%) were underweight, and 27,865 (11.1%) were overweight or obese. Of the 251,257 women, 18,612 (7.41%) had PTB delivery. Both low-and high-temperature exposure increased the risk of PTB in the third trimester, while cold exposure mostly mitigated the risk for the first and second trimesters. The association with low temperature was the strongest in the third trimester, especially for underweight women (HR: 1.825; 95%CI: 1.529 ~ 2.179), while the association with high temperature was the strongest also in the third trimester, especially for obese or overweight women (HR:1.825; 95%CI:1.502 ~ 2.218). Furthermore, the attributable fractions of PTB risk in the third trimester were estimated as 5.59% (95% CI: 3.58, 7.98%) for cold exposure among underweight women and 3.31% (95% CI: 2.01, 4.88%) for hot exposure among overweight or obese women.

Conclusion

Exposure to either low temperature in the third trimester or high temperature during pregnancy was associated with a higher risk of PTB. Moreover, pre-pregnancy BMI status might affect the susceptibility of pregnant women. Such findings would be useful to develop targeted measures for vulnerable populations.

Effects of ambient temperature and relative humidity on preterm birth during early pregnancy and before parturition in China from 2010 to 2018: a population-based large-sample cohort study

Background

The progression of global warming and increase in instances of extreme weather have received considerable attention. We conducted a cohort study on women of childbearing age in Yunnan Province, examined the association between ambient temperature and humidity on preterm birth and evaluated the effects of extreme weather during early pregnancy and before parturition on preterm birth.

Methods

We conducted a population-based cohort study on women of childbearing age 18-49 years who participated in National Free Preconception Health Examination Project (NFPHEP) in Yunnan Province from January 1, 2010, to December 31, 2018. Meteorological data, namely daily average temperature (°C) and daily average relative humidity (%), were obtained from China National Meteorological Information Center. Four exposure windows were explored: 1 week of pregnancy, 4 weeks of pregnancy, 4 weeks before delivery, and 1 week before delivery. We used a Cox proportional hazards model and adjusted the potential risk factors for preterm birth to obtain the effects of exposure to temperature and humidity on preterm birth among the stages of pregnancy.

Results

At 1 week of pregnancy and at 4 weeks of pregnancy, the association between temperature and preterm birth was U-shaped. The correlation between relative humidity and the risk of preterm birth was n-type at 1 week of pregnancy. The correlation between preterm birth and temperature and relative humidity at 4 weeks before delivery and at 1 week before delivery is J-shaped. Low temperature and low humidity were protective factors against preterm birth, whereas high temperature and high humidity were risk factors for preterm birth.The effects of high temperature and extremely high temperature were the strongest at 4 weeks before delivery, with HRs of 1.417 (95% CI: 1.362-1.474) and 1.627 (95% CI: 1.537-1.722), respectively. The effects of extremely low humidity and low humidity were strongest at 1 week before delivery, with HRs of 0.681 (95% CI: 0.609-0.761) and 0.696 (95% CI: 0.627-0.771), respectively.

Conclusion

Temperature and relative humidity affect preterm birth differently for each pregnancy stage. The effects of meteorological factors on pregnancy outcomes such as premature birth should not be ignored.

Maternal apparent temperature during pregnancy on the risk of offspring asthma and wheezing: effect, critical window, and modifiers

The objective of this study was to explore the impact of maternal AT during pregnancy on childhood asthma and wheezing, as well as the potential effect modifiers in this association. A cross-sectional study was implemented from December 2018 to March 2019 in Jinan to investigate the prevalence of childhood asthma and wheezing among aged 18 months to 3 years. Then, we conducted a case-control study based on population to explore the association between prenatal different AT exposure levels and childhood asthma and wheezing. The association was assessed by generalized additive models and logistic regression models, and stratified analyses were performed to explore potential effect modifiers. A total of 12,384 vaccinated children participated in screening for asthma and wheezing, 236 cases were screened, as well as 1445 controls were randomized. After adjusting for the covariates, childhood asthma and wheezing were significantly associated with cold exposure in the first trimester, with OR 1.731 (95% CI: 1.117-2.628), and cold exposure and heat exposure in the third trimester, with ORs 1.610 (95% CI: 1.030-2.473) and 2.039 (95% CI: 1.343-3.048). In the third trimester, enhanced impacts were found among girls, children whose distance of residence was close to the nearest main traffic road, and children whose parents have asthma. The study indicates that exposure to extreme AT during the first and third trimesters could increase the risk of childhood asthma and wheezing.

Synergic effects of PM1 and thermal inversion on the incidence of small for gestational age infants: a weekly-based assessment

BACKGROUND

The synergic effects of thermal inversion (TI) and particulate matter with an aerodynamic diameter ≤1 μm (PM₁) exposure and incidence of small for gestational age (SGA) was not clear.

OBJECTIVE

We aimed to explore the independent effects of prenatal TI and PM₁ exposure on incidence of SGA and their potential interactive effects.

METHODS

A total of 27,990 pregnant women who delivered in Wuhan Children's Hospital from 2017 to 2020 were included. The daily mean concentration of PM₁ was obtained from ChinaHighAirPollutants (CHAP) and matched with the residential address of each woman. Data on TI was derived from National Aeronautics and Space Administration (NASA). The independent effects of PM₁ and TI exposures on SGA in each gestational week were estimated by the distributed lag model (DLM) nested in Cox regression model, and the potential interactive effects of PM₁ and TI on SGA were investigated by adapting the relative excess risk due to interaction (RERI) index.

RESULTS

Per 10 μg/m³ increase in PM₁ was associated with an increase in the risk of SGA at 1-3 and 17-23 gestational weeks, with the strongest effect at the first gestational week (HR = 1.043, 95%CI: 1.008, 1.078). Significant links between one day increase of TI and SGA were found at the 1-4 and 13-23 gestational weeks and the largest effects were observed at the 17^th gestational week (HR = 1.018, 95%CI: 1.009, 1.027). Synergistic effects of PM₁ and TI on SGA were detected in the 20^th gestational week, with RERI of 0.208 (95%CI: 0.033,0.383).

IMPACT STATEMENT

Both prebirth PM₁ and TI exposure were significantly associated with SGA. Simultaneous exposure to PM₁ and TI might have synergistic effect on SGA. The second trimester seems to be a sensitive window of environmental and air pollution exposure.

Modification effects of ambient temperature on associations of ambient ozone exposure before and during pregnancy with adverse birth outcomes: A multicity study in China

BACKGROUND

Epidemiological studies suggest that both ambient ozone (O₃) and temperature were associated with increased risks of adverse birth outcomes. However, very few studies explored their interaction effects, especially for small for gestational age (SGA) and large for gestational age (LGA).

OBJECTIVES

To estimate the modification effects of ambient temperature on associations of ambient O₃ exposure before and during pregnancy with preterm birth (PTB), low birth weight (LBW), SGA and LGA based on multicity birth cohorts.

METHODS

A total of 56,905 singleton pregnant women from three birth cohorts conducted in Tianjin, Beijing and Maoming, China, were included in the study. Maximum daily 8-h average O₃ concentrations of each pregnant woman from the preconception period to delivery for every day were estimated by matching their home addresses with the Tracking Air Pollution in China (TAP) datasets. We first applied the Cox proportional-hazards regression model to evaluate the city-specific effects of O₃ exposure before and during pregnancy on adverse birth outcomes at different temperature levels with adjustment for potential confounders, and then a meta-analysis across three birth cohorts was conducted to calculate the pooled associations.

RESULTS

In pooled analysis, significant modification effects of ambient temperature on associations of ambient O₃ with PTB, LBW and LGA were observed (P_interaction < 0.05). For a 10 μg/m³ increase in ambient O₃ exposure at high temperature level (> 75^th percentile), the risk of LBW increased by 28 % (HR: 1.28, 95% CI: 1.13-1.46) during the second trimester and the risk of LGA increased by 116% (HR: 2.16, 95%CI: 1.16-4.00) during the entire pregnancy, while the null or weaker association was observed at corresponding low (≤ 25^th percentile) and medium (> 25^th and ≤ 75^th percentile) temperature levels.

CONCLUSION

This multicity study added new evidence that ambient high temperature may enhance the potential effects of ambient O₃ on adverse birth outcomes.

Interaction effects of night-time temperature and PM2.5 on preterm birth in Huai River Basin, China

Nocturnal temperature is observed increasing with global warming. However, evidence on night-time non-optimal temperature on the risk of preterm birth (PTB) is limited, and the potential interactions with air pollution on PTB has not been well clarified. We therefore conducted a population-based retrospective cohort study to evaluate the effect of night-time temperature extremes on the risk of PTB and its interaction with air pollution. Records of 196,780 singleton births from 4 counties in Huai River Basin (2013-2018) were obtained. Gridded data on night-time temperature were collected from a high-quality Chinese Air Quality Reanalysis dataset. We used a multivariate logistic regression to evaluate the effects of night-time heat and cold exposure on the risk of PTB as well as its subtypes. Potential interactions between night-time temperature extremes and fine particulate matter < 2.5 µm (PM_2.5) were examined using the relative excess risk due to interaction (RERI). We found that the risk of PTB was positively associated with third trimester night-time extremely heat and cold exposure, with adjusted OR of 1.898 (95 %CI: 1.655-2.177) and 2.044 (95 %CI: 1.786-2.339). Similar effects were observed for PTB subtypes, moderately PTB (mPTB) and very PTB (vPTB). Synergistic effects (RERI greater than 0) of each trimester night-time temperature extremes exposure and PM_2.5 on PTB were observed. We identified consistent positive interactions between night-time temperature extremes and PM_2.5 on mPTB. No significant interaction of night-time temperature extremes and PM_2.5 on vPTB was found. In conclusion, this large retrospective cohort study found that third trimester night-time heat and cold exposure significantly increased the risk of PTB and its subtypes. There is a synergistic effect between night-time temperature extremes and high PM_2.5 levels on PTB and mPTB. In the context of climate warming, our results add new evidence to the current understanding of night-time non-optimal temperature exposure on PTB.

The burden of heatwave-related preterm births and associated human capital losses in China

Frequent heatwaves under global warming can increase the risk of preterm birth (PTB), which in turn will affect physical health and human potential over the life course. However, what remains unknown is the extent to which anthropogenic climate change has contributed to such burdens. We combine health impact and economic assessment methods to comprehensively evaluate the entire heatwave-related PTB burden in dimensions of health, human capital and economic costs. Here, we show that during 2010-2020, an average of 13,262 (95%CI 6,962-18,802) PTBs occurred annually due to heatwave exposure in China. In simulated scenarios, 25.8% (95%CI 17.1%-34.5%) of heatwave-related PTBs per year on average can be attributed to anthropogenic climate change, which further result in substantial human capital losses, estimated at over $1 billion costs. Our findings will provide additional impetus for introducing more stringent climate mitigation policies and also call for more sufficient adaptations to reduce heatwave detriments to newborn.

Early delivery following chronic and acute ambient temperature exposure: a comprehensive survival approach

Background

Ambient temperature, particularly heat, is increasingly acknowledged as a trigger for preterm delivery but study designs have been limited and results mixed. We aimed to comprehensively evaluate the association between ambient temperature throughout pregnancy and preterm delivery.

Methods

We estimated daily temperature throughout pregnancy using a cutting-edge spatiotemporal model for 5347 live singleton births from three prospective cohorts in France, 2002–2018. We performed Cox regression (survival analysis) with distributed lags to evaluate time-varying associations with preterm birth simultaneously controlling for exposure during the first 26 weeks and last 30 days of pregnancy. We examined weekly mean, daytime, night-time and variability of temperature, and heatwaves accounting for adaptation to location and season.

Results

Preterm birth risk was higher following cold (5th vs 50th percentile of mean temperature) 7–9 weeks after conception [relative risk (RR): 1.3, 95% CI: 1.0–1.6 for 2°C vs 11.6°C] and 10–4 days before delivery (RR: 1.6, 95% CI: 1.1–2.1 for 1.2°C vs 12.1°C). Night-time heat (95th vs 50th percentile of minimum temperature; 15.7°C vs 7.4°C) increased risk when exposure occurred within 5 weeks of conception (RR: 2.0, 95% CI: 1.05–3.8) or 20–26 weeks after conception (RR: 2.9, 95% CI: 1.2–6.8). Overall and daytime heat (high mean and maximum temperature) showed consistent effects. We found no clear associations with temperature variability or heatwave indicators, suggesting they may be less relevant for preterm birth.

Conclusions

In a temperate climate, night-time heat and chronic and acute cold exposures were associated with increased risk of preterm birth. These results suggest night-time heat as a relevant indicator. In the context of rising temperatures and more frequent weather hazards, these results should inform public health policies to reduce the growing burden of preterm births.

Associations between ambient temperature and risk of preterm birth in Sweden: A comparison of analytical approaches

BACKGROUND

Evidence indicates that high temperatures are a risk factor for preterm birth. Increasing heat exposures due to climate change are therefore a concern for pregnant women. However, the large heterogeneity of study designs and statistical methods across previous studies complicate interpretation and comparisons. We investigated associations of short-term exposure to high ambient temperature with preterm birth in Sweden, applying three complementary analytical approaches.

METHODS

We included 560,615 singleton live births between 2014 and 2019, identified in the Swedish Pregnancy Register. We estimated weekly mean temperatures at 1-km² spatial resolution using a spatiotemporal machine learning methodology, and assigned them at the residential addresses of the study participants. The main outcomes of the study were gestational age in weeks and subcategories of preterm birth (<37 weeks): extremely preterm birth (<28 weeks), very preterm birth (from week 28 to <32), and moderately preterm birth (from week 32 to<37). Case-crossover, quantile regression and time-to-event analyses were applied to estimate the effects of short-term exposure to increased ambient temperature during the week before birth on preterm births. Furthermore, distributed lag nonlinear models (DLNM) were applied to identify susceptibility windows of exposures throughout pregnancy in relation to preterm birth.

RESULTS

A total of 1924 births were extremely preterm (0.4%), 2636 very preterm (0.5%), and 23,664 moderately preterm (4.2%). Consistent across all three analytical approaches (case-crossover, quantile regression and time-to-event analyses), higher ambient temperature (95th vs 50th percentile) demonstrated increased risk of extremely preterm birth, but associations did not reach statistical significance. In DLNM models, we observed no evidence to suggest an increased effect of high temperature on preterm birth risk. Even so, a suggested trend was observed in both the quantile regression and time-to-event analyses of a higher risk of extremely preterm birth with higher temperature during the last week before birth.

CONCLUSIONS

In Sweden, with high quality data on exposure and outcome, a temperate climate and good quality ante-natal health care, we did not find an association between high ambient temperatures and preterm births. Results were consistent across three complementary analytical approaches.

First Trimester of Pregnancy as the Sensitive Period for the Association between Prenatal Mosquito Coil Smoke Exposure and Preterm Birth

Mosquito coils are efficient mosquito repellents and mosquito coil smoke (MCS) contributes to indoor air pollution. However, no prior population-based study has investigated whether prenatal MCS exposure is a risk factor for preterm birth (PTB) and whether exposure to MCS in different trimesters of pregnancy is associated with different levels of risk. The sample involved 66,503 mother-child dyads. Logistic regression models were used to examine the relationships between prenatal MCS exposure during different trimesters of pregnancy and PTB. We found that prenatal MCS exposure was associated with a greater likelihood of PTB (OR = 1.12, 95%CI: 1.05-1.20). The prenatal MCS exposure during the first trimester was associated with 1.17 (95%CI: 1.09-1.25) times the odds of being PTB, which was higher than exposure during the second trimester (OR = 1.11, 95%CI: 1.03-1.19) and during the third trimester (OR = 1.08, 95%CI: 1.01-1.16). In the stratified analysis, prenatal MCS exposure significantly increased PTB risk among girls but not among boys. Our results indicated that maternal MCS exposure during pregnancy was associated with PTB and that the first trimester might be the sensitive period. In light of these findings, public health interventions are needed to reduce prenatal exposure to MCS, particularly during the first trimester of pregnancy.

Critical window for the association between prenatal environmental tobacco smoke exposure and preterm birth

Although environmental tobacco smoke (ETS) exposure is considered to be a severe public health problem and a modifiable risk factor for preterm birth (PTB), we still lack a comprehensive understanding of the PTB risk associated with trimester-specific prenatal ETS exposure. This study aimed to examine the accumulation of risk across trimester ETS exposure and the critical window of the association between maternal ETS exposure during pregnancy and PTB. A total of 63,038 mother-child pairs were involved in the analysis of the 2017 survey of Longhua Child Cohort Study. Information about socio-demographic characteristics, prenatal ETS exposure, and birth outcomes were collected using a self-report questionnaire. A series of logistic regression models were employed to assess the associations between prenatal ETS exposure and PTB. We found that maternal ETS exposure during pregnancy significantly increased the risk of PTB and this association increased with both the average level of daily ETS exposure and the number of trimesters of ETS exposure. Moreover, mothers who were initially exposed to ETS in the 1st trimester of pregnancy had significant higher risk of PTB (OR = 1.34, 95% CI: 1.25-1.44). Furthermore, mothers exposed to ETS in the 1st trimester only (OR = 1.26, 95%CI: 1.04-1.50), in both 1st and 2nd trimester (OR = 1.35, 95%CI: 1.08-1.67) and throughout pregnancy (OR = 1.35, 95%CI: 1.24-1.46) experienced a significantly high risk of PTB. Prenatal maternal ETS exposure during only the 2nd trimester also resulted in a high risk of PTB with marginal significance (OR = 1.33, 95% CI:0.78-2.13). To conclude, the 1st and early 2nd trimester might be the critical window for prenatal ETS exposure causing PTB.

Effects of extreme temperature on the risk of preterm birth in China: A population-based multi-center cohort study

BACKGROUND

Extreme temperatures are associated with the risk of preterm birth (PTB), but evidence on the effects of different clinical subtypes and across different regions is limited. We aimed to evaluate the effects of maternal exposure to extreme temperature on PTB and its clinical subtypes in China, and to identify effect modification of regional factors in dimensions of population, economy, medical resources and environmental factors.

METHODS

This was a prospective population-based cohort of 210,798 singleton live births from 16 counties in eight provinces across China during 2014-2018. We used an extended Cox regression with time-varying variables to evaluate the effects of extreme heat and cold on PTB and its subtypes in the entire pregnancy, each trimester, the last gestational month and week. Meta-analysis and meta-regression were conducted to estimate the pooled effects of each city and effect modification by regional characteristics.

FINDINGS

Exposure to heat and cold during the entire pregnancy significantly increased the risk of PTB. The effects varied with subtypes, for medically indicated and spontaneous PTB, hazard ratios were 1·84 (95% CI: 1·29, 2·61) and 1·50 (95% CI: 1·11, 2·02) for heat, 2·18 (95% CI: 1·83, 2·60) and 2·15 (95% CI: 1·92, 2·41) for cold. The associations were stronger for PTB less than 35 weeks than those during weeks 35-36. The effects varied across locations, and GDP per capita (β=-0·16) and hospital beds per 1000 persons (β=-0·25) were protective factors for the effects.

INTERPRETATION

Extreme temperature can increase the risk of medically indicated and spontaneous PTB, and higher regional socio-economic status may moderate such effects. In the context of climate change, such findings may have important implications for protecting the health of vulnerable groups, especially newborns.

FUNDING

National Key R&D Program of China (2018YFA0606200), National Natural Science Foundation of China (42175183), Strategic Priority Research Program of the Chinese Academy of Sciences (XDA20030302), National Natural Science Foundation of China (42071377).

Exposure to heat during pregnancy and preterm birth in North Carolina: Main effect and disparities by residential greenness, urbanicity, and socioeconomic status

BACKGROUND

Although previous literature suggested that several factors may be associated with higher risk of adverse health outcomes related to heat, research is limited for birth outcomes.

OBJECTIVES

We investigated associations between exposure to heat/heat waves during the last week of gestation and preterm birth (PTB) in North Carolina (NC) and evaluated effect modification by residential greenness, urbanicity, and socioeconomic status (SES).

METHODS

We obtained individual-level NC birth certificate data for May-September 2003-2014. We estimated daily mean temperature at each maternal residential address using Parameter-elevation Regressions on Independent Slopes Model (PRISM) data. We created 3 definitions of heat waves (daily temperature ≥95th, 97th, 99th percentile for NC warm season temperature, for ≥2 consecutive days). Normalized Difference Vegetation Index (NDVI) was used to assess residential greenness. Community-level modifiers (e.g., income, urbanicity) were considered. We applied Cox proportional hazard models to estimate the association between exposure to heat/heat waves and PTB, controlling for covariates. Stratified analyses were conducted to evaluate whether the association between heat and PTB varied by several individual and community characteristics.

RESULTS

Of the 546,441 births, 8% were preterm. Heat exposure during the last week before delivery was significantly associated with risk of PTB. The hazard ratio for a 1 °C increase in temperature during the last week before delivery was 1.01 (95% CI: 1.00, 1.02). Higher heat-PTB risk was associated with some characteristics (e.g., areas that were urbanized, low SES, or in the Coastal Plain). We also found significant PTB-heat risk in areas with low greenness for urbanized area. For heat waves, we did not find significantly positive associations with PTB.

DISCUSSION

Findings provide evidence that exposure to heat during pregnancy increases risk of PTB and suggest disparities in these risks. Our results have implications for future studies of disparity in heat and birth outcomes associations.

Effect of Elevated Ambient Temperature on Maternal, Foetal, and Neonatal Outcomes: A Scoping Review

This scoping review provides an overview of the published literature, identifies research gaps, and summarises the current evidence of the association between elevated ambient temperature exposure during pregnancy and adverse maternal, foetal, and neonatal outcomes. Following the PRISMA extension for scoping reviews reporting guidelines, a systematic search was conducted on CINAHL, PubMed, and Embase and included original articles published in the English language from 2015 to 2020 with no geographical limitations. A total of seventy-five studies were included, conducted across twenty-four countries, with a majority in the USA (n = 23) and China (n = 13). Study designs, temperature metrics, and exposure windows varied considerably across studies. Of the eighteen heat-associated adverse maternal, foetal, and neonatal outcomes identified, pre-term birth was the most common outcome (n = 30), followed by low birth weight (n = 11), stillbirth (n = 9), and gestational diabetes mellitus (n = 8). Overall, papers reported an increased risk with elevated temperature exposures. Less attention has been paid to relationships between heat and the diverse range of other adverse outcomes such as congenital anomalies and neonatal mortality. Further research on these less-reported outcomes is needed to improve understanding and the effect size of these relationships with elevated temperatures, which we know will be exacerbated by climate change.

The Association of Ambient Temperature with Extremely Preterm Births

INTRODUCTION

Extremely preterm births (EPT), require complex decision making and significant medical interventions. While environmental factors such as ambient temperature extremes have been associated with preterm births, little is known of the environmental associations with EPT births. The objective of this study is to explore whether ambient temperature is associated with increased risk of EPT birth.

METHODS

Birth records for 315,226 infants born in Queensland Australia (2007-2015) were matched to average maximum and minimum temperature for the last month of pregnancy. Odds ratios and 95% confidence intervals were calculated using a generalised linear model. Population attributable risk was calculated for a 5% reduction in maximum temperature.

RESULTS

Each one degree increase in maximum [aOR 1.03 (95% CI 1.01, 1.05)] and minimum temperature [aOR 1.02 (95% CI 1.01, 1.04)] was associated with an increase in odds for EPT birth. Increased odds for EPT births was found for maternal smoking [aOR 1.46 (95% CI 1.23, 1.72)], increasing plurality [OR 6.38 (95% CI 5.48, 7.42)] and stillbirth [aOR 342.99 (95% CI 295.53, 398.06)]. When stratified by birth status, the association was only found for live births.

DISCUSSION

Higher temperatures are associated with small increases in the odds of delivering an infant in the EPT period. The risk may be enhanced for women who smoke during pregnancy. Women at an increased risk of preterm births should be counselled around methods to reduce their exposure to excessive heat.

Impact of ambient temperature exposure on newborns with low Apgar scores in northwest China

In the context of global climate change, research efforts were focused on the association of ambient temperatures on maternal and neonatal health condition, but few have examined associations with low Apgar scores. From January 1, 2017, to December 31, 2018, all singleton deliveries of Ningxia Hui Autonomous Region were extracted from the Hospital Information System (N = 182,322). Daily temperature data were obtained from the official website of China Meteorological Administration. Low Apgar scores were defined as Apgar score ≤ 3 at 5 min in the present study. Logistic regression models were used to estimate the adjusted association between prenatal temperature exposure and low Apgar scores. Restricted cubic spline models were used to explore the dose-response relationship between temperature and low Apgar scores. The study population included 182,322 live singleton births, with 1575 (0.86%) cases of low Apgar scores. The elevated ambient temperature in different exposure timing windows in late pregnancy was associated with increased risk of low Apgar scores. As compared to moderate (10th-90th) temperature exposure, prenatal exposure to extreme hot (>90th) was associated with 13.9-47.0% increased risk of low Apgar scores, while non-significant relationship was found between extreme cold (<10th) exposure and low Apgar scores. The restricted cubic spline models showed a U-shaped relationship between prenatal temperature exposure and low Apgar scores (P for non-linearity < 0.05). Exposure to high ambient temperature during late pregnancy is associated with an increased risk of low Apgar scores in northwest China.

Effect of heat waves and fine particulate matter on preterm births in Korea from 2010 to 2016

BACKGROUND

Previous studies have reported that fine particulate matter (PM_2.5) affects the incidence of premature births. In addition, recent studies have suggested that heat waves have a negative impact on birth outcomes. However, the combined effect of PM_2.5 and heat waves on the incidence of premature birth is controversial. This study investigated the independent and combined effects of PM_2.5 and heat wave exposures during the 1st and 2nd trimesters on premature birth.

METHODS

The National Statistical Office of Korea provided birth data from 2010 to 2016. Preterm birth was defined as birth between 22 and 36 weeks. To assess the exposure to PM_2.5 and heat waves, we used PM_2.5 data estimated by the Community Multiscale Air Quality Modeling System (CMAQ) and heat wave warning data provided by the Korea Meteorological Administration. A multivariate logistic regression was used to investigate the risk of preterm birth according to the exposure to PM_2.5 and heat waves during the 1st and 2nd trimesters, and it was adjusted for residential area, year of birth, season of birth, parity, education level of the mother, age of the mother, and sex of the baby.

RESULTS

In the 2nd trimester, compared with the 0 h of heat wave exposure (≤67 percentile), 62.50-314.00 h (79-88 percentile) and>315.00 h of heat wave exposure (>88 percentile) were both significantly associated with preterm birth (OR for 79-88 percentile, 1.037, 95% CI, 1.003-1.073; OR for > 88 percentile, 1.174, 95% CI, 1.134-1.215). However, PM_2.5 exposure was not significantly associated with preterm birth. On the other hand, in the analysis to evaluate the combined effect of PM_2.5 and heat wave exposures of the 2nd trimester, compared with 0 h of heat wave exposure (≤67 percentile) and<11.64 μg/m³ (≤25 percentile) of PM_2.5, 11.64-22.74 μg/m³ (≤25 percentile), 22.74-27.58 μg/m³ (26-50 percentile), and 27.57-32.39 μg/m³ (51-75 percentile) of PM_2.5 exposure combined with>315.00 h of heat wave exposure (>88 percentile) were all significantly associated with preterm birth. In addition, the effect size was increased with an increase of PM_2.5 exposure (OR for ≤ 25 percentile, 1.148, 95% CI, 1.095-1.203; OR for 26-50 percentile, 1.248, 95% CI, 1.178-1.323; OR for 51-75 percentile, 1.370, 95% CI, 1.245-1.507).

CONCLUSION

Our findings suggest that the combined effect of heat wave and PM_2.5 exposure during the 2nd trimester on the risk of preterm birth was greater than that of each exposure alone. In other words, exposure to PM_2.5 increases the impact of heat waves on the risk of preterm birth. These results indicate that control of prenatal exposure to fine particular matter and extreme temperatures is important for the prevention of preterm birth.