## 1 Introduction  
Birth weight is commonly used as a measure of fetal growth. The World Health Organization (WHO) defines low birth weight (LBW) as any live birth weighing less than 2500 g [1]. LBW has been recognized as an important risk factor for fetal and neonatal mortality and morbidity, delayed growth and cognitive development, and development of several chronic diseases later in life [2]. It has been estimated that 15% to 20% of all births worldwide are LBW, i.e., more than 20 million births a year [3]. In Finland, the prevalence of low birth weight is approximately 4.3% [4,5]. Humans are surrounded by air pollution throughout their life; however, the prenatal period is possibly the most vulnerable period. During this period, children are especially susceptible to adverse effects of environmental exposures [6]. The evidence on the impact of prenatal exposure to air pollutants on fetal growth and development has been summarized in several systematic reviews and meta-analyses; however, the results and conclusions have been inconsistent and even controversial [6; 7; 8; 9; 10]. [PERSON] et al. [11] presented already two decades ago associations between outdoor air pollution and the risk of term LBW in Northeastern cities of the United States. Their study found that maternal exposure to SO2 and CO during the second and third trimesters, respectively, increase the risk of LBW. [PERSON] et al. [12] compiled recently data from 14 prospective birth cohorts involving 12 European countries (i.e., European Study of Cohorts for Air Pollution Effects (ESCAPE)). An increase of 5 ug/m3 in prenatal exposure to PM2.5 and PM10 was associated with 18% and 16% increased risk of LBW, respectively. Even exposure to PM2.5 concentrations below the limit values recommended by the European Union were associated with an increased risk of LBW. Several meta-analyses have summarized the effects related to exposure to PM on the risk of adverse pregnancy outcomes [6; 8; 13; 14]. [PERSON] et al. [9] reported summary-effect estimates for reduced birth weight, ranging from 11.4 g per 1 ppm of CO to 28.1 g per 20 ppb of NO2. In a recent original study from our team on the effect of air pollution on preterm birth (PTB), we found an increased risk of PTB with maternal exposure to relatively low levels of air pollution. We also found synergistic effects of PM2.5 and ozone [15]. The study by [PERSON] et al. [12] on the effect of air pollution included several European countries but did not include Finland. Based on our systematic literature search, the present study is the first to investigate potential effects of air pollution on LBW in a cold subarctic climate (also called subpolar or boreal climate) present in Finland. In general, concentrations of the most relevant air pollutants in the Helsinki region have been found to be lower than the corresponding values in major Central and Southern European cities [16]. However, pollutant levels in Finland may be substantially higher in unfavorable weather conditions in winter, spring, and summer. Emissions into the air are generated from residential wood combustion, vehicular traffic, resuspended dust, energy production, and industry, especially in urban areas. In addition, long-range transport has a notable contribution to air pollutant concentrations in Finland, including smoke episodes from forest fires [17]. Although there was substantial evidence on the effects of prenatal exposure to air pollution on fetal growth, there were no previous studies conducted in cold climate with relatively low levels of air pollution. None of the previous studies have elaborated potential interactions between different pollutants or potential seasonal differences in their effects on fetal growth. To fill these gaps in knowledge, we assessed potential effects of maternal exposure to air pollutants during pregnancy on the risk of LBW in a cold climate.