A 20-year-old parturient woman has the I labor stage. The pregnancy is full-term. Labors occur every 3 minutes and last for 55 seconds. Fetus presentation is polar, the head is pressed to the small pelvis entrance. Heart rate of the fetus is 150/min, distinct and rhythmic. Vagina examination: uterus cervix is smoothed out; mouth of the womb is 2 cm open; fetal bladder is intact; the head is presented over the I plane of small pelvis; moderate mucous-bloody discharge is observed. What phase of the I labor stage is it?