A 28-year-old primigravida at term is in labor. At 4 hours after admission, cervical examination shows 5 cm dilation with regular contractions every 2 minutes and intact membranes. At 6 hours, she has 7 cm dilation with contractions every 90 seconds and spontaneous rupture of membranes (clear fluid). Which finding best distinguishes her transition into the second stage of labor from her current active phase status?