Seasonal patterns and preterm birth: a systematic review of the literature and an analysis in a London-based cohort

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.

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.

Ambient air pollution and preterm birth: a time-series analysis

BACKGROUND

An emerging body of evidence suggests that ambient levels of air pollution during pregnancy are associated with preterm birth.

METHODS

To further investigate these relationships we used vital record data to construct a retrospective cohort of 476,489 births occurring between 1994 and 2004 in 5 central counties of metropolitan Atlanta. Using a time-series approach, we examined aggregated daily counts of preterm birth in relation to ambient levels of carbon monoxide, nitrogen dioxide, sulfur dioxide, ozone, particulate matter <10 microm in diameter (PM10), particulate matter <2.5 microm in diameter (PM2.5), and speciated PM measurements. Daily pollutant levels in 5-county Atlanta were characterized using a population-weighted spatial average of air quality monitors in the study area. We also examined ambient concentrations at individual monitors in analyses limited to mothers with residential geocodes within 4 miles of each monitor. Relationships between average pollution levels during 3 gestational windows of interest were modeled using Poisson generalized linear models. Results were adjusted for seasonal and long-term time trends.

RESULTS

Although most results were null, there were 3 positive associations between ambient pollution levels and preterm birth in the 4-mile capture-area analyses. Daily preterm birth rates were associated with average NO2 concentrations in the preceding 6 weeks and with average PM2.5 sulfate and PM2.5 water-soluble metal concentrations in the preceding week.

CONCLUSIONS

Results provide limited support for late-pregnancy effects of ambient air pollution on preterm birth.

Impact of intensive care practices on short-term and long-term outcomes for extremely preterm infants: comparison between the British Isles and France

OBJECTIVES

The objective of this study was to compare practices of care and outcomes of infants who were born between 23 and 25 weeks' gestation in 1995 in the British Isles and in 1997-1998 in France.

METHODS

We examined 2 population-based cohorts in the British Isles (1892 births included) and in France (456 births): the EPICure and EPIPAGE studies. The rate of follow-up was 90% at 30 months and 86% at 2 years. At 5 to 6 years, the cognitive function of 64% of the children without severe disability was assessed in the EPICure study and 57% in the EPIPAGE study.

RESULTS

The mortality rate of live-born infants was lower in the EPICure study (25%) than in the EPIPAGE study (34%) before admission to a NICU but higher in the NICU (45% vs 29%, respectively), such that there was no difference in the proportions of survivors at discharge after adjustment for gestational age. The risk for severe brain lesions was 24% among infants who were admitted to a NICU in both studies, 41% in the EPICure study versus 67% in the epidemiologic study on great prematurity (EPIPAGE) among infants who died after discontinued treatment in NICU, and 17% vs 11% among survivors at discharge. The risk for cerebral palsy at 24 to 30 months was 20% in the EPICure study versus 16% in the EPIPAGE study, whereas the risk for overall cognitive score of <70 at 5 to 6 years was 10% vs 14%, respectively.

CONCLUSIONS

Despite apparent differences in the modalities of limitation of intensive care, the outcomes of infants who were born at 23 to 25 weeks' gestation in the EPICure and EPIPAGE studies were not significantly different.

The prevalence of preterm birth and season of conception

Preterm birth is a major obstetric problem. An exploration of the season of conception in relation to preterm birth may provide direction in the search for risk factors. We conducted a retrospective cohort study of 82 213 singleton livebirths (20-45 weeks' gestation) to 61,630 women at Magee-Womens Hospital, Pittsburgh, PA, from 1995 to 2005. Conception was estimated based on gestational age determined by best obstetric estimate. Fourier series analysis was used to model seasonal trends. Spontaneous preterm birth at <37 weeks was associated with conception season (P < 0.05). The peak prevalence occurred among conceptions in winter and spring (peaking February 23 at 6.9%), with an average trough among late summer/early autumn conceptions (August 25 at 6.2%). The pattern for spontaneous preterm birth <32 weeks was similar (P < 0.05), with the peak on March 13 (1.7%), and nadir on September 12 (1.4%). Results were similar when indicated preterm births were included. These seasonal changes may increase our insight into the role of exposures with seasonal periodicity in the pathophysiology of preterm birth.

Meeting report: atmospheric pollution and human reproduction

BACKGROUND

There is a growing body of epidemiologic literature reporting associations between atmospheric pollutants and reproductive outcomes, particularly birth weight and gestational duration.

OBJECTIVES

The objectives of our international workshop were to discuss the current evidence, to identify the strengths and weaknesses of published epidemiologic studies, and to suggest future directions for research.

DISCUSSION

Participants identified promising exposure assessment tools, including exposure models with fine spatial and temporal resolution that take into account time-activity patterns. More knowledge on factors correlated with exposure to air pollution, such as other environmental pollutants with similar temporal variations, and assessment of nutritional factors possibly influencing birth outcomes would help evaluate importance of residual confounding. Participants proposed a list of points to report in future publications on this topic to facilitate research syntheses. Nested case-control studies analyzed using two-phase statistical techniques and development of cohorts with extensive information on pregnancy behaviors and biological samples are promising study designs. Issues related to the identification of critical exposure windows and potential biological mechanisms through which air pollutants may lead to intrauterine growth restriction and premature birth were reviewed.

CONCLUSIONS

To make progress, this research field needs input from toxicology, exposure assessment, and clinical research, especially to aid in the identification and exposure assessment of feto-toxic agents in ambient air, in the development of early markers of adverse reproductive outcomes, and of relevant biological pathways. In particular, additional research using animal models would help better delineate the biological mechanisms underpinning the associations reported in human studies.

The impact of nonlinear exposure-risk relationships on seasonal time-series data: modelling Danish neonatal birth anthropometric data

Background

Birth weight and length have seasonal fluctuations. Previous analyses of birth weight by latitude effects identified seemingly contradictory results, showing both 6 and 12 monthly periodicities in weight. The aims of this paper are twofold: (a) to explore seasonal patterns in a large, Danish Medical Birth Register, and (b) to explore models based on seasonal exposures and a non-linear exposure-risk relationship.

Methods

Birth weight and birth lengths on over 1.5 million Danish singleton, live births were examined for seasonality. We modelled seasonal patterns based on linear, U- and J-shaped exposure-risk relationships. We then added an extra layer of complexity by modelling weighted population-based exposure patterns.

Results

The Danish data showed clear seasonal fluctuations for both birth weight and birth length. A bimodal model best fits the data, however the amplitude of the 6 and 12 month peaks changed over time. In the modelling exercises, U- and J-shaped exposure-risk relationships generate time series with both 6 and 12 month periodicities. Changing the weightings of the population exposure risks result in unexpected properties. A J-shaped exposure-risk relationship with a diminishing population exposure over time fitted the observed seasonal pattern in the Danish birth weight data.

Conclusion

In keeping with many other studies, Danish birth anthropometric data show complex and shifting seasonal patterns. We speculate that annual periodicities with non-linear exposure-risk models may underlie these findings. Understanding the nature of seasonal fluctuations can help generate candidate exposures.