All sample size and power estimates are based on two-tailed tests. This is important because we will be powered to detect both increases and decreases in outcomes with immediate compared to delayed pushing. The baseline spontaneous vaginal delivery rate with delayed pushing used for the sample size estimation is based on our observational cohort. We estimate that a total of 3184 (1592 delayed and 1592 immediate pushing) will be sufficient to detect a 5% absolute difference (estimated 72% versus 77%) in spontaneous vaginal delivery with 90% power (alpha of 0.05).