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The prevalence of back pain was studied in 855 pregnant women who were followed from the 12th week of pregnancy, every 2nd week, until childbirth. The 9-month period prevalence was 49%, with a point prevalence of 22-28% from the 12th week until delivery. Because 22% of the women had back pain at the 12th week of the pregnancy, the 6-month incidence was 27%. Based on pain drawings, back pain was classified into three groups: In one group, pain was localized to the sacroiliac areas and increased as pregnancy progressed; in the other two groups, pain either decreased or did not change, respectively. True sciatica with a dermatomal distribution occurred in only 10 women (1%). Back problems before pregnancy increased the risk of back pain, as did young age, multiparity, and several physical and psychological work factors.