Background: The prevalence of fear of birth has been estimated between 8-30%, but there is considerable heterogeneity in research design, definitions, measurement tools used and populations. There are some inconclusive findings about the stability of childbirth fear. Aim: to assess the prevalence and characteristics of women presenting with scores >= 60 on FOBS-The Fear of Birth Scale, in mid and late pregnancy, and to study change in fear of birth and associated factors. Methods: A prospective longitudinal cohort study of a one-year cohort of 1212 pregnant women from a northern part of Sweden, recruited in mid pregnancy and followed up in late pregnancy. Fear of birth was assessed using FOBS-The fear of birth scale, with the cut off at >= 60. Findings: The prevalence of fear of birth was 22% in mid pregnancy and 19% in late pregnancy, a statistically significant decrease. Different patterns were found where some women presented with increased fear and some with decreased fear. The women who experienced more fear or less fear later in pregnancy could not be differentiated by background factors. Conclusions: More research is needed to explore factors important to reduce fear of childbirth and the optimal time to measure it.