A 26-year-old secundipara at 40 weeks of gestation arrived at a maternity ward after the beginning of labor activity. The bursting of waters occurred 2 hours prior. The fetus was in a longitudinal lie with cephalic presentation. Abdominal circumference was 100 cm, fundal height - 42 cm. Contractions occurred every 4-5 minutes and lasted 25 seconds each. Internal obstetric examination revealed cervical effacement, opening by 4 cm. Fetal bladder was absent. Fetal head was pressed against the pelvic inlet. What complication arose in the childbirth?