Secular trends in seasonal variation in birth weight
Introduction
Both low and high birth weights (BW) are characteristics that besides being the major determinant of fetal mortality and morbidity has been shown to be associated with a wide range of adult traits and health outcomes [1], [2], [3].
Both genetic and environmental factors have been shown to influence BW [4]. Gestational age, parity, maternal age, gestational weight gain, maternal smoking, diet and life style during pregnancy, parental body size (BW, adult height and BMI) and occupation has been associated with BW [4]. A better understanding of the determinants may help enabling identification of high-risk groups for both high and low BW.
Season of birth has also been associated with BW, and numerous studies have shown that mean BW varies across seasons [5], [6], [7], [8], [9], [10], [11], [12]. Identified seasonal patterns include both 1 and 2 annual peaks, and season of BW peaks and lows (nadirs) is inconsistent between sites [13]. It has been speculated that the heterogeneity in the seasonal patterns is a reflection of the difference in latitudes between the study sites [14]. Also, differences in analytical strategy might be responsible. No obvious interpretation of the seasonal variation in BW has been recognized, but several mechanisms have been proposed [6], [10], [13].
One of the proposed mechanisms behind seasonal variation in BW is that BW follows the seasonal fluctuations in vitamin D [9]. Vitamin D is synthesized in the skin when exposed to ultraviolet B (UVB) radiation from the sun, and the amount of synthesized vitamin D therefore depends on the season [15]. The hypothesis is that exposure to vitamin D during pregnancy ensures proper fetal development and growth, and that vitamin D deficiency during pregnancy may lead to reduced BW as a function of alterations in the fetal development. If the majority of the pregnancy has been experience during the colder months (October to April in Denmark) the synthesis of vitamin D would have been minimal leaving the mother and the child at an increased risk of vitamin D insufficiency (assuming that they did not take vitamin D supplements). If low vitamin D is associated with lower BW, then children born during the early spring would be expected to have the lowest BWs and children born during the fall would have the highest BWs.
Two previous studies have investigated seasonal variation in BW in Danish children [10], [16]. None of these have looked at the seasonal pattern before birth year 1973, or across a longer time period (maximum time period 30 years (10)), and they have not investigated the potential determinants of the varying BW.
The aim of the present study was to investigate the seasonal variation in BW in Denmark during a 53 year period (1936–1989). An additional aim was to see if this variation could be explained by the fluctuations in sunshine hours.
Section snippets
Methods
Information on BW was collected from the Copenhagen School Health Records Register (CSHRR) which contains information on BW of all children who went to school in the Copenhagen Municipality born from 1936 to 1989. Validation of BW information in CSHRR against medical birth records has shown a high validity of the information (in preparation). At school entry the mother or father reported the BW of the child. The cohort contains 372 636 records and it is described in details elsewhere [17].
Results
We identified 326 520 children born during 1936 to 1989. The total number of children included in the study was 276 339 (50.77 % boys) after exclusion of 48 313 children with missing BW information, 1 007 children with BWs below 1.5 kg, and 860 children with BWs above 5.5 kg. The number of children born per year ranged from 2 243 in 1983 to 10 697 in 1946 (Fig. 1).
Mean BW in the study population was 3353.4 (SD 568) grams, and mean BW was not stable across the period. There was a decreasing trend
Discussion
In the present study we identified a seasonal variation in BW that was not stable across the 53 years of study. In the first 20 years of study we observed a fall peak in BW, but gradually over time this peak diminished and was taken over by an early summer peak. The lowest BW month also changed over time from an early summer low in the first 10 years of study to a winter low in the remaining study period. Our results clearly showed that the seasonal pattern in BW was non-stationary since both the
Acknowledgements
We would like to acknowledge Adrian Barnett for his support regarding the use of the R package Season.
The present study was funded by the Danish Agency for Science Technology and Innovation, the Ministry of Science, Innovation and Higher Education, under the instruments “Strategic Research Projects” and by a research grant from the Danish PhD School of Molecular Metabolism funded by the Novo Nordisk Foundation. The supporting bodies had no role in the design, implementation, analysis and
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