A 23-year-old woman, para 2, full term, is in labor. Her waters were clear and broke three hours ago. The labor activity is regular. The contractions last 25-30 seconds, with intervals of 4-5 minutes. The baby is in the longitudinal lie, cephalic presentation. The head is pressed to the entrance into the lesser pelvis, Fetal heart rate is 136/min. Internal examination shows that the cervix is smoothed out, the opening of the external orifice of uterus is 3 cm, no amniotic sac, the lower pole of the fetal head is at the level of terminalis. What stage of labor is it?