Global warming is one of the emerging concerns for the 21 st century and is increasing in such a way that environmental temperature has been risen about 0/5-1°C since the mid-21 st-century.[ 1] Strong scientific documents suggest that climate and weather changes are quickly rising. Human activities especially fossil fuels burning is one the important resources of global warming.[ 2] Children and infants are vulnerable people who are most at risk of climate changes related effects because of physical and physiological condition and lack of recognition maturation.[ 3] Environmental hazards quickly change and increase in result of the increase in climate change and harm to younger children and infants more than others.[ 4] Preterm birth (PTB) and low birth weight (LBW) are two important problems that may lead to mortality and other birth diseases and can affect significantly on other problems and diseases in future life of child.[ 5] Although hazards of climate changes for human health are known[ 6] but extent and amount of these hazards and its consequences especially on pregnancy are not yet recognized. Given the importance of the effects of climate change on pregnancy outcomes, and in turn the lifelong effects of adverse pregnancy outcomes; the present study has considered the effects of climate changes on pregnancy outcomes including LBW and PTB using a systematic review.
The search process was conducted in electronic databases including papers published until June 2014 in ISI Web of knowledge, PubMed, Scopus, and Google Scholar using key words of "environmental temperature," "pregnancy," "LBW," "pregnancy outcome," "climate change," "PTB," and a combination of them. To extract related articles in PubMed, medical subject heading was used. We considered cross-sectional, case-control, and longitudinal studies. Editorials, commentaries, and letter to editor papers were not included. The search was refined to the English language; we did not consider any time limitation. For any additional pertinent studies, the reference list of all relevant papers was screened as well.
Titles and abstracts of papers were screened, and relevant papers were selected. Duplicates were removed. Then, full texts of relevant papers were read, and findings were rescreened. Two independent (PP and MK) reviewers screened titles and abstracts of papers identified by the literature searches for their potential relevance or assessed the full text for inclusion in the review. In the case of disagreement, and the discrepancy was resolved in consultation with a third arbitrating investigator (RK).
Two reviewers abstracted the data independently. The required information that was extracted from all eligible papers was as follows: Data on first author's last name, year of publication and country of the study population, the study name, study design characteristics, study population, type of study, aim and finding of studies.
Articles were excluded after reviewing their abstracts, and finally 15 articles[ 7 8 9 10 11 12 13 14 15 16 17 18 19 20] were included.
Effects of climate change on pregnancy outcomes were considered. Figure 1 presents the study flowchart. Table 1 shows the data extracted from articles included in this review. Different studies were conducted around the world including Asia, Europe, and America; most of them were from America and Europe. The studies were performed according to different methods and principles. Effects of the temperature, different seasons of fertilization time, season of childbirth, humidity and month of childbirth were assessed in these studies. Many diverse measurable outcomes were considered, e.g., eclampsia, preeclampsia, cataract, LBW, PTB, hypertension, sex ratio and length of pregnancy. Approximately all studies have reported a significant relationship between exposure variable and intended outcome. According to conducted studies, decrease in birth weight is more possible in cold months and winter. Increase in temperature was followed by increase in PTB rate. PTB was higher in exposure of the January to February months compared to March to October months.
Flowchart of selecting primary studies for the systematic review
Summary of studies on the effects of climate change on birth outcomes
Studies had presented different results about eclampsia and preeclampsia; however, according to most of the studies, they were more prevalent in cold and humid seasons. Some studies have reported higher risk in winter and some in summer. About the effect of month and season of fertilization on eclampsia and preeclampsia, it was shown that fertilization in the summer might increase the risk of these two conditions compared to fertilization in fall and winter.
This systematic review assessed the relationship of climate change to some pregnancy outcomes. Different studies have measured effects of these changes on eclampsia, preeclampsia, cataract, LBW, PTB, hypertension, sex ratio, and length of pregnancy. In general, review of studies suggested harmful effects of the climate and temperature changes on human health. It must be noted that two temperature spectrums either cold or hot influenced health. In some studies, cold weather and in other hot weather were associated with health outcomes. About cataract, the hot weather and 5° increase in the temperature were associated with a cataract occurrence. In the study of Catalano et al., cold weather was associated with an increase in the sex ratio and life duration of male sex in 1 st year. Cold weather might produce these effects through different mechanisms including direct thermal effects, nutritional shortage and higher exposure to internal contaminants. Dadvand et al. studied hot weather or thermal stress and pregnancy length, they found that exposure to thermal stress was associated with 5 days decrease in the average pregnancy age. Secretion of oxytoxin and heat-shock proteins in results of heat stress is increased; it is recognized as one of the factors related to PTB.[ 21 22 23] One of the other possible reasons of this consequence related to thermal exposure might be because of dehydration.[ 24]
Exposure to cold weather and birth in winter and fall that is, cold seasons, is reported to be associated with LBW and very LBW. PTB is found to be associated with two spectrums of weather extent (cold and hot). Exposure to cold weather, cold storm and high heat has been related to PTB. Increase in hydrosols, viscosity, blood vessel and blood cholesterol have been stated as possible factors.[ 25 26] This relationship has been more frequent in mothers with younger age, black race, and Asian background. It is documented that hypertension, eclampsia, and preeclampsia were more frequent in cold weather; cold seasons with more wind and rain. In addition, fertilization in the summer might increase the chance of developing preeclampsia. In addition, increased sunlight intensity has been associated with a decrease in blood pressure. Factors like the effects of Vitamin D, increase in temperature, ultraviolet light, and seasonal infections are mentioned as possible factors.[ 27 28 29] Increasing number of studies related to weather and temperature changes highlights the importance of these changes in human health especially on mothers and infants.
Above review included some limitations such as lack of homogeneity between studies, different methodologies, different sample size and variations in the studied populations.
Diverse studies have been conducted about effects of different aspects of climate change including the temperature, season, rain and wind on human health and their results have documented different adverse health effects of climate change on human health. The findings of the current review confirm the crucial importance of the harmful effects of climate changes in the perinatal period.
Source of Support: Nil
Conflict of Interest: None declared.