Data analysis Power calculation To show a statistically significant difference of at least 20 mm in pain intensity (considered as a clinically relevant difference) measured by visual analogue scale between the active treatment group and the control group, at least 32 women will be needed with the assumptions of type I error of 5% and type II error of 20%. To take account for data loss the investigators aim at a sample size of at least 30 women in each group, in total 60 women.