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Changing seasonal variation in births by sociodemographic factors: a population-based register study
Stockholm University, Faculty of Social Sciences, Department of Sociology.ORCID iD: 0000-0002-4952-3959
Stockholm University, Faculty of Social Sciences, Department of Sociology.
2018 (English)In: Human Reproduction Open, ISSN 2399-3529, no 4, article id hoy015Article in journal (Refereed) Published
Abstract [en]

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 pronounced amonghighly educated mothers. Over the 72 years studied, the seasonal variation among first-time mothers declined steadily and has almost disappearedat the end of the study period. Using data that cover ~180 000 births in each month, all meaningful results are statistically significant. LIMITATIONS REASONS FOR CAUTION: The study uses data from one Nordic country only, making it difficult to draw conclusionsthat may hold for other countries. WIDER IMPLICATIONS OF THE FINDINGS: The typical seasonal variation reported for Sweden between 1940 and 1999, with highnumbers of births during the spring and low numbers of births during the last quarter of the year, is in line with results from most otherEuropean countries during the same time period. However, the significant decline in seasonal variation in the early 21st century is a noveldevelopment. The study underlines that in a society with low fertility and efficient birth control, active choices and behaviours associated withan individual’s sociodemographic characteristics tend to matter more for the seasonal timing of childbearing than environmental factorsrelated to the physiological ability to reproduce and cultural–behavioural factors related to the frequency of intercourse.

Place, publisher, year, edition, pages
2018. no 4, article id hoy015
Keywords [en]
Birth seasonality, Fertility, Socioeconomic factors, Register data, Family planning
National Category
Sociology (excluding Social Work, Social Psychology and Social Anthropology)
Research subject
Demography
Identifiers
URN: urn:nbn:se:su:diva-164954DOI: 10.1093/hropen/hoy015OAI: oai:DiVA.org:su-164954DiVA, id: diva2:1280869
Available from: 2019-01-21 Created: 2019-01-21 Last updated: 2019-12-12Bibliographically approved

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