Breeding for a larger litter size has affected the vitality of piglets negatively. The larger litter sizes lead to lower individual birthweight. Smaller pigs have bigger difficulties to keep up their body temperature right after birth because of their lower muscle mass. Other factors that affect the vitality of the piglets are season, how the pigs are cared for and how they are housed. The purpose of the study is to analyze changes in litter size and birthweight over the time period 2013 to 2017. The study also analyses how litter size and birthweight is affected by season. The study performs statistical analyses on data recorded at the pig facilities at Lövsta agriculture research facility, SLU in Uppsala, Sweden, between the years 2013 to 2017. This study includes piglets of the crossbreed Yorkshire x Hampshire. The questions of issue in this study concerns the change in litter size and birthweight over time years 2013 to 2017, the regression between litter size and birthweight and the effect of season on birthweight and litter size. The study showed that the birthweight decreased over the years. Regression showed that the increase of litter size lowers the birthweight. The effect of season was significant indicating lower birthweights in the second compared to the fourth quarter of the year. The effect of season can potentially be explained by weather conditions. Keywords: litter size, birth weight, newborn pigs/piglets, sow, animal welfare.