A 35-year-old woman, gravida 1, is at 39 weeks of her full-term pregnancy. The duration of the second stage of labor is 1.5 hours. Objectively, the dimensions of the pelvis are 26-29-32-20, the expected weight of the fetus - 4400 g, contractions are active, the fetus is in a cephalic presentation with the head pressed to the entrance to the lesser pelvis, Henkel-Vasten sign is positive. The fetal heart rate is 118-126/min, sonorous, rhythmic. Vaginal examination detects complete opening of the cervix, the head is one finger-width in the lesser pelvis, severe caput succedaneum and edema of the vulva are observed. What complication has occurred during the childbirth?