WU Lei, DING Yi, RUI Xing Li, and MAO Cai Ping

Seasonal Variations in Birth Weigh t in Suzhou lndustrial Park＊

WU Lei1,2,^, DING Yi1,2,^, RUI Xing Li3, and MAO Cai Ping1,#

Many environmental factors have been shown to adversely influence birth weight, and new insight has been gained into ‘seasonal programm ing'. We studied a total of 23,064 infants. The mean birth weight varied across seasons. Logistic regression analysis was used to obtain the crude and adjusted odds ratios (ORs) for dichotomous outcomes (e.g., macrosom ia, low birth weight). There were significant differences in the risks for macrosom ia in infants born in different seasons. Compared w ith those for infants born in spring, the ORs for macrosom ia were 0.85 [95% confidence interval (CI): 0.75-0.98] and 0.87 (95% CI: 0.77-0.99) for infants born in summer and autumn, respectively. These findings suggest that environmental factors may have public health implications and should be considered when primary prevention programs are developed for macrosom ia or low birth weight.

Increasing epidem iological data have shown that adverse environmental insults (e.g., low-protein diets, nicotine use, high-sucrose diets, and others) during pregnancy may ‘program' the development of adult diseases (e.g., diabetes mellitus, hypertension, cancer, and others)[1-2]. Birth weight is an important indicator of infant mortality, and it affects health outcomes in both childhood and adulthood[1,3]. Abnormal size at birth, including both low birth weight and macrosom ia, has been associated w ith increased susceptibility to diseases in later life, causing great personal and societal burdens. Increased risks for cardiovascular diseases[4-7]and diabetes mellitus[8]in later life were reported to be associated with low birth weight, while an increased risk of cancer was found to be correlated w ith a higherbirthweight[9].Bothgeneticand environmental factors have been shown to influence birth weight. The fragile biochem ical stability of the intrauterine environment during pregnancy is significantly influenced by environmental factors, such as high or low temperatures, with considerable effects on fetal grow th and infant development. It is well known that a fetus has a lim ited ability to regulate its temperature and that such regulation depends largely on the mother's thermoregulatory capacity. Hitherto, different reports worldw ide have gained attention regarding the effect of ‘seasonal programm ing'. However, there is considerable controversy about which season is associated w ith abnormal birth weight.

Early investigations in developed countries found no significant differences among seasons in birth weights[10-11]. In contrast, subsequent studies of large populations found statistically significant seasonal fluctuations in birth weight[12-13]. We have the opportunity to study seasonal fluctuations in birth weight in Suzhou, China, which has a hum id, subtropical climate characterized by hot, hum id summers and m ild, cool winters. The primary objective of the present investigation was to determ ine whether birth weight fluctuated by season for infants born in Suzhou between 2006 and 2010.

The study was approved by the Suzhou Industrial Park Centers for Disease Control and Prevention Ethics Comm ittee. Information regarding 23,064 infants born full-term (at least 37 weeks and＜42 weeks gestation) between January 2006 and December 2010 at hospitals in Suzhou Industrial Park, Jiangsu Province, was obtained via public birth records. The information included date of birth, sex, birth weight (g), and gestational age (weeks). Information for infants who died after birth also was included.

A logistic regression analysis was used to obtain the crude and adjusted odds ratios (ORs) for dichotomous outcomes (macrosom ia and low birthweight). Previous studies showed a U-shaped relationshipbetweenbirthweightand adult-programmed diseases[14-15]. Therefore, because the relationship between weight and risk is not linear, including infants w ith macrosom ia and infants w ith low birth weight in the same data set may cause risk effects to be obscured. Hence, we used two separate databases for analyses: a database for infants w ith low birth weight (＜2500 g) and a database for infants w ith macrosom ia (≥4000 g). Initial analyses that incorporated cases categorized into months of birth demonstrated common patterns based on the different seasons of the year. Thus, all data were re-analyzed according to date of birth using categorizations for spring (March-May), summer(June-August),autumn (September-November),andwinter (December-February). Data were presented as means±standard deviations (SDs) for continuous variables, and percentages for categorical variables. Data analyses were conducted using SPSS Statistics 21. A P value of ＜0.05 was considered statistically significant.

A total of 23,064 (52.80% boys) infants were studied. The mean birth weight of the study population was 3400.60±416.06 g. The average gestational age was 39.27±1.18 weeks. Overall, 90.30% of infants were first-borns, and 39.70% were delivered via caesarean section (Table 1). The average birth weight was significantly higher in spring and w inter. In the contrast, there was no significant difference between summer and autumn w ith respect to birth weight (Table 2).

The present study demonstrated that the risk of macrosom ia varied by season. The ORs for macrosom ia were 0.85 [95% confidence interval (CI): 0.75-0.98] and 0.87 (95% CI: 0.77-0.99) for infants born in the summer and autumn, respectively, compared to infants born in the spring. However, compared w ith infants born in spring, the OR for macrosom ia was 1.00 (95% CI: 0.88-1.14) for infants born in the winter (Table 3). As shown in Table 4, compared with infants born in the spring, the OR for low birth weight was 0.75 (95% CI: 0.56-1.11) for infants born in the summer. Sim ilarly, compared w ith infants born in the spring, the ORs for low birth weight were 0.98 (95% CI: 0.70-1.20) and 0.82 (95% CI: 0.61-1.10) for infants born in the autumn and w inter, respectively (Table 4). Seasonal variation was not associated w ith a risk for low birth weight. The major strength of our study is that it included a large, population-based sample. We used a logistic regression analysis to determ ine the effect of seasonal variations. ORs were used to estimate which season protected against macrosom ia, and we compared them w ith a chi-squared analysis. To our know ledge, this is the first study reporting the effects of ‘seasonal programm ing' on birth weight in China.

Table 1. Characteristics of Study Subjects Born at Full-term Gestation by Season of Birth

Table 2. Distributions of Gestational Age and Birth Weight by Season of Birth

Table 3. Distributions of the Subjects and the Results of the Logistic Regression Analyses for Macrosom ia by Season at Birth

Table 4. Distributions of the Subjects and the Results of the Logistic Regression Analyses for Low Birth Weight by Season of Birth

In the present study, we identified that the average birth weight in spring and w inter was significantly higher than that in summer and autumn. Flouris et al. found that there was a negative correlation between ambient temperature during the month of birth and birth weight[16]. Previous studies demonstrated that seasonal variation in birth weight was associated with seasonal fluctuations in vitam in D[17]. Prenatal exposure to vitam in D ensures fetal grow th and development, and vitam in D deficiency during pregnancy may reduce birth weight.

It is well known that the delivery process for fetuses with macrosom ia is associated w ith a high risk of health hazards for both the mother and infant. Chodick et al. found that infants born in Israel in the summer had an increased risk for macrosom ia compared with those born in w inter[18]. However, the present research showed that infants born during the summer and autumn seasons in Suzhou had a significantly decreased risk for macrosom ia.

The present study had some lim itations. We did not collect and analyze objective factors, such as the mother's health consciousness and exposure to sunshine, which, in addition to seasonal factors, m ight have influenced birth weight. Future research should exam ine these variables.

In conclusion, the influence of seasonal variation on birth weight varies by ethnicity and probably has various underlying mechanisms or results. Birth during the summer and autumn seasons was associated with a decreased risk for macrosom ia in Suzhou, and a low birth weight in others cities. These findings suggest that environmental factors may have public health implications and should be considered when primary prevention programs are developed for macrosom ia or low birth weight.

^These authors contributed equally to this work.

#Correspondence should be addressed to MAO Cai Ping, E-mail: maocp1018@163.com

Accepted: October 1, 2016

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10.3967/bes2016.101

*This work was partially supported by grants from the National Nature Science Foundation of China (81370719); Suzhou Science & Technology Project (SYS201405); and Suzhou Key Technologies of Prevention and Control of Major Disease and Infectious Diseases (Gwzx201506).

1. Institute for Fetology and Reproductive Medicine Center, First Hospital of Soochow University, Suzhou 215006, Jiangsu, China; 2. Suzhou Industrial Park Centers for Disease Control and Prevention, Suzhou 215021, Jiangsu, China; 3. Nanjing First Hospital, Nanjing 210029, Jiangsu, China

Biographical notes of the s: WU Lei, female, born in 1984, Doctor, majoring in prenatal medicine; DING Yi, female, born in 1986, Doctor, majoring in epidem iology and health statistics.

June 17, 2016;