Calculation of sample size
We assumed that the non-OA (OP and OT) prevalence during the first stage
of labor was 60%. We hypothesized that the rate of spontaneous internal
rotation to the OA position would be 80% in the group, and the
proportions of cesarean delivery in the non-OA position and OA position
were 30% and 10%, respectively. The total estimated sample size was
507 by accepting an alpha risk of 0.05 (two-sided) and a power of 80%.
Finally, we increased the number by 20% (634 women) to compensate for
the dropout rate and any secondary exclusion.