Changing seasonal variation in births by sociodemographic factors : a population-based register study

STUDY QUESTION: Have seasonal variations in births by factors related to maternal education, age, parity and re-partnering changedover a 72-year period? SUMMARY ANSWER: Seasonal variation in births has been reduced overall but also changed its pattern over the last seven decades. WHAT IS KNOWN ALREADY: The number of births varies markedly by season, but the causes of this variation are not fully understood.Seasonality of births is, in some populations, strongly influenced by sociodemographic factors. STUDY DESIGN SIZE, DURATION: A longitudinal study design was used by analysing the seasonal variation in live births between 1940and 2012, and relating it to mothers’ sociodemographic characteristics at the time of childbirth (maternal education, age, parity and repartnering). PARTICIPANTS/MATERIALS, SETTING, METHODS: Register data on 6 768 810 live births in Sweden between 1940 and 2012were used. Information on biological parents are available for more than 95% of all births. Multinomial logistic regressions were used to calculatepredicted probabilities of giving birth for each calendar month. MAIN RESULTS AND THE ROLE OF CHANCE: Between 1940 and 1999, Swedish birth rates showed the typical seasonal variationwith high numbers of births during the spring, and low numbers of births during the last quarter of the year. However, during the 21st century,the seasonal variation in fertility declined so that only minor variation in birth rates between February and September now remains. Still, thepattern of low birth rates at the end of the year remains and has even become more pronounced from the 1980s onwards. The characteristic‘Christmas effect’ that used to be visible in September has vanished over the last 30 years. The roles in seasonal variation of maternal education,the mother’s age, parity and instances where the mother has re-partnered between subsequent births changed during the second half ofthe 20th century. From 1980s onwards, the decline in birth rates during the last quarter of the year became particularly ...