Changing Seasonal Variation in Births by Sociodemographic Factors: A Population-Based Register Study

Abstract: Seasonality of births is in some populations strongly influenced by sociodemographic factors. In this study, we analyse the impact of mothers’ sociodemographic characteristics for the seasonal variation in 7,710,955 live births in Sweden between 1940 and 2012. During 1940-1999, Swedish birth rates showed the typical seasonal variation with high numbers of births during the spring, and low numbers of births during the last quarter of the year. However, during the twenty-first century, the seasonal variation in fertility declined so that only minor variation in birth rates between February and September remains. Still, the pattern of low birth rates at the end of the year remains and has even become more pronounced in recent decades. The roles of maternal education, mother’s birth country, parity, and instances where the mother has re-partnered between subsequent births changed during the second half of the twentieth century. The study underlines that in a society with low fertility and efficient birth control, active choices and behaviours associated with an individual’s sociodemographic characteristics tend to matter more for the seasonal timing of childbearing than environmental factors related to the physiological ability to reproduce and cultural-behavioural factors related to the frequency of intercourse.