• Ingibjorg Eiríksdottir, Elinborg J Olafsdottir, Laufey Tryggvadottir, and Thora Steingrimsdottir.
• Department of Obstetrics and Gynecology, Landspitali- The National University Hospital, Iceland.
• Laeknabladid. 2023 Dec 1; 109 (12): 551558551-558.

IntroductionWorldwide, the health-promoting effects of breastfeeding on children and their mothers are indisputable. The frequency and duration of breastfeeding varies greatly internationally but studies on prevalence and influencing factors of breastfeeding in Iceland are scanty and the published ones deal with small groups. The aim of this research is to describe the epidemiology of breastfeeding duration and its influencing factors in Iceland among a large cohort in a whole population over almost one century.Material And MethodsThis is a historical cohort study, using data from The Cancer Detection Clinic Cohort of The Icelandic Cancer Society, collected retrospectively by questionnaires during the years 1964-2008. The data consisted of mothers´ reported information on breastfeeding of their 81,889 children, 36,537 first-borns and 45,352 younger siblings. The frequency and duration of breastfeeding was calculated and the effects of the following exposure variables were assessed: Maternal age, BMI (N=4950, data collected 1979-2008) and smoking (N=32.087, data collected 1995-2008), the child's year of birth and its order in the sibling group.ResultsThe children were born in the period of 1917-2008. In the late 1970s, the average duration of breastfeeding began to increase, in all age groups of mothers, increasing rapidly from 3 months to 7-8 months. At about the same time, the breastfeeding duration increased depending on the birth order of the children, younger children were breastfed for longer than their older siblings. Women with normal weight (BMI 18.5 to 24.9) breastfed their babies the longest, while obese women breastfed the shortest. Women with any history of smoking reported shorter duration of breastfeeding than women who had never smoked.ConclusionThe increase in the average duration of breastfeeding in Iceland a few decades ago is in accordance with the information in the World Health Organization's database from European countries 1975-2000, where the Nordic countries and most Northern European countries promoted breastfeeding at a similar time. High BMI and maternal smoking are important variables when studying breastfeeding and this study indicates their negative association with the duration of breastfeeding.

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