Correction per 30 April 2018. In a letter from Ole Olsen et al [1] it was disclosed, that there were mathematical errors in the incidences of complications among nulliparous women with hospital births presented in Table 2. Incidences and p-values have been recalculated, and the errors in Table 2 have been corrected. We apologise for this. The corrections did not lead to any other changes in the manuscript [1].
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