A time-series analysis of any short-term effects of meteorological and air pollution factors on preterm births in London, UK

Associations of meteorology with adverse pregnancy outcomes: a systematic review of preeclampsia, preterm birth and birth weight

The relationships between meteorology and pregnancy outcomes are not well known. This article reviews available evidence on the relationships between seasonality or meteorology and three major pregnancy outcomes: the hypertensive disorders of pregnancy (including preeclampsia, eclampsia and gestational hypertension), gestational length and birth weight. In total 35, 28 and 27 studies were identified for each of these outcomes. The risks of preeclampsia appear higher for women with conception during the warmest months, and delivery in the coldest months of the year. Delivery in the coldest months is also associated with a higher eclampsia risk. Patterns of decreased gestational lengths have been observed for births in winter, as well as summer months. Most analytical studies also report decreases in gestational lengths associated with heat. Birth weights are lower for deliveries occurring in winter and in summer months. Only a limited number of studies have investigated the effects of barometric pressure on gestational length or the effects of temperature and sunshine exposure on birth weight, but these questions appear worth investigating further. Available results should encourage further etiological research aiming at enhancing our understanding of the relationships between meteorology and adverse pregnancy outcomes, ideally via harmonized multicentric studies.

Effect of ambient temperature and air pollutants on the risk of preterm birth, Rome 2001-2010

INTRODUCTION

Although the prevalence of preterm births ranges from 5 to 13% and represents the leading cause of perinatal mortality and morbidity in developed countries, the etiology of preterm birth remains uncertain. We aimed to evaluate the effect of short-term exposure to high and low temperatures and air pollution on preterm delivery and to identify socio-demographic and clinical maternal risk factors enhancing individual susceptibility.

METHODS

We analyzed all singleton live births by natural delivery that occurred in Rome in 2001-2010. A time-series approach was used to estimate the effect of exposure to minimum temperature, maximum apparent temperature, heat waves, particulate matter with an aerodynamic diameter of 10μm or less (PM10), ozone, and nitrogen dioxide in the month preceding delivery; the analysis was conducted separately for cold and warm seasons. Socio-demographic and clinical risk factors were included as interaction terms.

RESULTS

Preterm births comprised nearly 6% of our cohort. An increase of 1.9% (95% confidence interval (CI) 0.86-2.87) in daily preterm births per 1°C increase in maximum apparent temperature in the 2days preceding delivery was estimated for the warm season. Older women, women with higher education levels, and women with obstetric or chronic pathologies reported during delivery had a lower effect of temperature on the risk of preterm birth, while women with a chronic disease in the two years before delivery and mothers<20years showed a higher effect. A +19% (95% CI 7.91-31.69) increase in preterm births was observed during heat waves. Temperature had no effect during the cold season. We detected a significant effect of PM10 on preterm-birth risk at a lag period of 12-22days during the warm season (+0.69%; 95% CI 0.23-1.15, for 1μg/m(3) increase of pollutant); women with obstetric pathologies and with a higher education level showed a higher risk.

CONCLUSIONS

Our results suggest a possible short-term effect of heat and a more delayed and prolonged effect of PM10 exposure on preterm-birth risk, as well as the existence of more susceptible subgroups of women. Our observations support the few reported investigations, and may help to increase awareness among public-health stakeholders and clinicians regarding the role of these environmental exposures as risk factors for premature birth and health consequences for children later in life.

High environmental temperature and preterm birth: a review of the evidence

OBJECTIVE

to examine the evidence in relation to preterm birth and high environmental temperature.

BACKGROUND

this review was conducted against a background of global warming and an escalation in the frequency and severity of hot weather together with a rising preterm birth rate.

METHODS

electronic health databases such as: SCOPUS, MEDLINE, CINAHL, EMBASE and Maternity and Infant Care were searched for research articles, that examined preterm birth and high environmental temperature. Further searches were based on the reference lists of located articles. Keywords included a search term for preterm birth (preterm birth, preterm, premature, <37 weeks, gestation) and a search term for hot weather (heatwaves, heat-waves, global warming, climate change, extreme heat, hot weather, high temperature, ambient temperature). A total of 159 papers were retrieved in this way. Of these publications, eight met inclusion criteria.

DATA EXTRACTION

data were extracted and organised under the following headings: study design; dataset and sample; gestational age and effect of environmental heat on preterm birth. Critical Appraisal Skills Programme (CASP) guidelines were used to appraise study quality.

FINDINGS

in this review, the weight of evidence supported an association between high environmental temperature and preterm birth. However, the degree of association varied considerably, and it is not clear what factors influence this relationship. Differing definitions of preterm birth may also add to lack of clarity.

KEY CONCLUSIONS

preterm birth is an increasingly common and debilitating condition that affects a substantial portion of infants. Rates appear to be linked to high environmental temperature, and more especially heat stress, which may be experienced during extreme heat or following a sudden rise in temperature. When this happens, the body may be unable to adapt quickly to the change. As global warming continues, the incidence of high environmental temperature and dramatic temperature changes are also increasing. This situation makes it important that research effort is directed to understanding the degree of association and the mechanism by which high temperature and temperature increases impact on preterm birth. Research is also warranted into the development of more effective cooling practices to ameliorate the effects of heat stress. In the meantime, it is important that pregnant women are advised to take special precautions to avoid heat stress and to keep cool when there are sudden increases in temperature.

Air pollution exposure in early pregnancy and adverse pregnancy outcomes: a register-based cohort study

OBJECTIVES

Our aim was to study the possible associations between exposure to elevated levels of air pollution, ozone (O(3)) and vehicle exhaust (NO(x)), during early gestation, and adverse pregnancy outcomes such as pre-eclampsia, preterm birth and small for gestational age.

DESIGN

Prospective register-based cohort study.

SETTING

The Swedish Medical Birth Register includes data on all deliveries during 1998 to 2006 in Greater Stockholm, Sweden. The national Patient Register and the Prescribed Drug Register were used to collect information on maternal asthma.

PARTICIPANTS

All singleton pregnancies, conceived at the earliest in August 1997 and at the latest in February 2006, were included, n=120 755.

OUTCOME MEASURES

We studied preterm birth, small for gestational age and pre-eclampsia.

RESULTS

4.4% of pregnancies resulted in a preterm birth. The prevalence of pre-eclampsia was 2.7%. We observed an association between first trimester O(3) and preterm birth (OR 1.04, 95% CI 1.01 to 1.08) as well as an association with pre-eclampsia (OR 1.04, 95% CI 1.01 to 1.08), per 10 µg/m(3) increase in O(3). We observed no association between first trimester NO(x) and adverse pregnancy outcomes. No associations were observed between any of the air pollutants and small for gestational age.

CONCLUSIONS

Increased levels of O(3) during the first trimester increased the risk of pre-eclampsia and preterm birth. Air pollutants did not exhibit any effects on fetal growth restriction. We estimated 1 in every 20 cases of pre-eclampsia to be associated with O(3) exposure.

The association of season and temperature with adverse pregnancy outcome in two German states, a time-series analysis

Background

A seasonality of low birth weight (LBW) and preterm birth (PTB) has been described for most regions and there is evidence that this pattern is caused by ambient outdoor temperature. However, the association as such, the direction of effect and the critical time of exposure remain controversial.

Methods

Logistic, time-series regression was performed on nearly 300,000 births from two German states to study the association between season and daily mean temperature and changes in daily proportions of term LBW (tLBW) or PTB. Analyses were adjusted for time-varying factors. Temperature exposures were examined during different periods of pregnancy.

Results

Weak evidence for an association between season of conception, season of birth or ambient outdoor temperature and tLBW or PTB was found. Results of analyses of temperature were not consistent between the two states. Different sources of bias which would have artificially led to stronger findings were detected and are described.

Conclusions

No clear evidence for an association between season of conception, season of birth or temperature and tLBW or PTB was found. In the study of pregnancy outcome different sources of bias can be identified which can potentially explain heterogeneous findings of the past.

Time-to-event analysis of fine particle air pollution and preterm birth: results from North Carolina, 2001-2005

Exposure to air pollution during pregnancy has been suggested to be a risk factor for preterm birth; however, epidemiologic evidence remains mixed and limited. The authors examined the association between ambient levels of particulate matter <2.5 μm in aerodynamic diameter (PM(2.5)) and the risk of preterm birth in North Carolina during the period 2001-2005. They estimated the risks of cumulative and lagged average exposures to PM(2.5) during pregnancy via a 2-stage discrete-time survival model. The authors also considered exposure metrics derived from 1) ambient concentrations measured by the Air Quality System (AQS) monitoring network and 2) concentrations predicted by statistically fusing AQS data with process-based numerical model output (the Statistically Fused Air and Deposition Surfaces (FSD) database). Using the AQS measurements, an interquartile-range (1.73 μg/m(3)) increase in cumulative PM(2.5) exposure was associated with a 6.8% (95% posterior interval: 0.5, 13.6) increase in the risk of preterm birth. Using the FSD-predicted levels and accounting for prediction error, the authors also found significant adverse associations between trimester 1, trimester 2, and cumulative PM(2.5) exposure and preterm birth. These findings suggest that exposure to ambient PM(2.5) during pregnancy is associated with increased risk of preterm birth, even in a region characterized by relatively good air quality.

Maternal exposure to ambient temperature and the risks of preterm birth and stillbirth in Brisbane, Australia

Almost 10% of all births are preterm, and 2.2% are stillbirths. Recent research has suggested that environmental factors may be a contributory cause of these adverse birth outcomes. The authors examined the relation between ambient temperature and preterm birth and stillbirth in Brisbane, Australia, between 2005 and 2009 (n = 101,870). They used a Cox proportional hazards model with livebirth and stillbirth as competing risks. They also examined whether there were periods in pregnancy where exposure to high temperatures had a greater effect. Higher ambient temperatures in the last 4 weeks of the pregnancy increased the risk of stillbirth. The hazard ratio for stillbirth was 0.3 at 12°C relative to the reference temperature of 21°C. The temperature effect was greatest at less than 36 weeks of gestation. There was an association between higher temperature and shorter gestation, as the hazard ratio for livebirth was 0.96 at 15°C and 1.02 at 25°C. This effect was greatest at later gestational ages. These results provide strong evidence of an association between increased temperature and increased risk of stillbirth and shorter gestation.

Preterm birth seasonality in Greece: an epidemiological study

OBJECTIVE

Seasonality of preterm birth has been noted, although not conclusively studied. Weather is also thought to play a role. We sought preterm birth seasonality and additionally studied the effect of weather parameters in the preterm birth pattern.

METHODS

Vital statistics from the Hellenic Statistical Authority were retrieved, covering the years from 1980 to 2008. Additionally, weather data were retrieved for the years of the study. Time series analysis was used to create various statistical models that would be compared to each other for their accuracy to predict preterm birth. Factors used in the modeling included month of birth, gender and weather factors.

RESULTS

Preterm birth seasonality was exhibited. Two peaks of higher risk of preterm birth were noted: One during summer and one during winter. Males were more influenced by seasonality and exhibited slightly different seasonal patterns than females, although no higher risk for preterm birth was noted. The best model that described seasonal pattern of preterm birth was the one that included meteorological factors. Notably, extreme (hotter or colder) weather was accompanied by an increase in preterm birth.

CONCLUSIONS

Evidence for seasonality of preterm birth was shown and extreme weather was associated with a higher incidence of it.

The influence of season and ambient temperature on birth outcomes: a review of the epidemiological literature

Seasonal patterns of birth outcomes, such as low birth weight, preterm birth and stillbirth, have been found around the world. As a result, there has been an increasing interest in evaluating short-term exposure to ambient temperature as a determinant of adverse birth outcomes. This paper reviews the epidemiological evidence on seasonality of birth outcomes and the impact of prenatal exposure to ambient temperature on birth outcomes. We identified 20 studies that investigated seasonality of birth outcomes, and reported statistically significant seasonal patterns. Most of the studies found peaks of preterm birth, stillbirth and low birth weight in winter, summer or both, which indicates the extremes of temperature may be an important determinant of poor birth outcomes. We identified 13 studies that investigated the influence of exposure to ambient temperature on birth weight and preterm birth (none examined stillbirth). The evidence for an adverse effect of high temperatures was stronger for birth weight than for preterm birth. More research is needed to clarify whether high temperatures have a causal effect on fetal health.

High ambient temperature and the risk of preterm delivery

With temperatures expected to increase because of climate change, it is essential to study the health outcomes of elevated temperature in vulnerable populations, such as expectant mothers. In this study, the authors estimated the association between heat and humidity, as measured by apparent temperature, and preterm delivery. They conducted a case-crossover analysis of almost 60,000 births spanning 16 counties in California that occurred from 1999 to 2006 between May and September. The authors identified cases of preterm birth from a state registry of births, which were combined with meteorologic and air pollution monitoring data based on residential zip code. High ambient temperature was significantly associated with preterm birth for all mothers, regardless of maternal racial/ethnic group, maternal age, maternal education, or sex of the infant. Results indicated that an 8.6% increase (95% confidence interval: 6.0, 11.3) in preterm delivery was associated with a 10°F (5.6°C) increase in weekly average (lag06) apparent temperature. Greater associations were observed for younger mothers, blacks, and Asians. These associations were independent of air pollutants. Given the significant associations for apparent temperature and preterm delivery found in this study, more large-scale studies of temperature and preterm delivery are warranted.

Does area level socioeconomic status modify the effects of PM(10) on preterm delivery?

BACKGROUND

Air pollution has been one of the focal points for investigations of maternal health and birth, and an association has been found between air pollution and birth outcomes. Some individual-level risk factors of mothers have been identified as strong predictors of preterm delivery (PTD). Recent literature has shown that the neighborhood environment of the mother and child has an independent influence on birth outcomes that was not explained by individual-level risk factors.

OBJECTIVE

This study explored the effects of maternal exposure to particulate air pollution (PM(10)) and its relationship to the probability of preterm delivery (PTD) in Seoul, Korea. Our hypothesis was that PM(10) increased the probability of PTD, and that the area level socioeconomic status (SES), measured by monthly average household income, modified the PM(10) effects on PTD after controlling for individual-level risk factors such as individual-level SES.

DATA AND METHOD

Birth data from the Korean National Statistics Office consisted of records on 433,173 singleton births in the years 2000-2003, and included individual characteristics. Area-level characteristics were also controlled for in 25 administrative regions in Seoul. Maternal exposure to PM(10) was calculated at an individual level for each trimester using information from monitoring sites in residential areas. Hourly data on PM(10) for 1999-2003 were acquired from the National Institute of Environment Research. Birth outcome was dichotomously coded: term birth (gestation > or =37 weeks) or PTD (gestation <37 weeks). For multilevel analysis, we conducted a hierarchical logistic regression with a random intercept for each administrative area to account for the hierarchical nature of the data.

RESULTS

We found that area-level SES modified the PM(10) effects on PTD, specifically among those who were in low SES areas. Results of multilevel analyses showed that an increase in PM(10) by 10 microg/m(3) led to a statistically significant increase in the probability of PTD [3.12% (95% CI, 0.17-6.15)] during the 2nd trimester in the low-income group. Analyses using the stratified population found a 3.4% increase (95% CI, 0.31-6.58) in the probability of PTD during the 2nd trimester in areas with low income.

CONCLUSION

Mothers exposed to higher PM(10) in Seoul, Korea, had a higher probability of PTD. Individual-level characteristics had different effects on the probability of PTD depending on the area-level SES. Area-level SES modified the effect of PM(10) concentration on PTD.

Seasonality of birth and implications for temporal studies of preterm birth

BACKGROUND

A strength of time-series analyses is the inherent control of individual-level risk factors that do not vary temporally. However, in studies of adverse pregnancy outcomes, risk factors considered time-invariant at the individual level may vary seasonally when aggregated into a pregnancy risk set. To illustrate, we describe the seasonal patterns of birth in Atlanta and demonstrate how these patterns could lead to confounding in time-series studies of seasonally-varying exposures and preterm birth.

METHODS

The study cohort included all births in 20-county metropolitan Atlanta delivered during the period 1994-2004 (n = 715,875). We assessed the seasonal patterns of estimated conception and birth for the full cohort and for subgroups stratified by sociodemographic factors. Based on the observed patterns, we quantified the degree of potential confounding created by (1) differences in the gestational age distribution in the risk set across calendar months and (2) differences in the sociodemographic composition of the risk set across calendar months.

RESULTS

The overall seasonal pattern of birth was characterized by a peak in August-September and troughs in April-May and November-January. Seasonal patterns differed among racial and ethnic groups, maternal education levels, and marital status. As a consequence of these seasonal patterns, systematic seasonal differences in the gestational age distribution and the sociodemographic composition of the risk set led to differences in expected rates of preterm birth across calendar months.

CONCLUSIONS

Time-series investigations of seasonally-varying exposures and adverse pregnancy outcomes should consider the potential for bias due to seasonal heterogeneity in the risk set.

Air pollution and infant health: Lessons from New Jersey

We examine the impact of three "criteria" air pollutants on infant health in New Jersey in the 1990s by combining information about mother's residential location from birth certificates with information from air quality monitors. Our work offers three important innovations. First, we use the exact addresses of mothers to select those closest to air monitors to improve the accuracy of air quality exposure. Second, we include maternal fixed effects to control for unobserved characteristics of mothers. Third, we examine interactions of air pollution with smoking and other risk factors for poor infant health outcomes. We find consistently negative effects of exposure to carbon monoxide (CO), both during and after birth, with effects considerably larger for smokers and older mothers. Since automobiles are the main source of carbon monoxide emissions, our results have important implications for regulation of automobile emissions.