A 27-year-old woman, gravida 1, para 1, was hospitalized into the maternity ward. She had a 3-year-long history of primary infertility Contractions started 9 hours ago, occur every 4-5 minutes, and last 20-25 seconds The waters broke 2.5 hours ago. The fetal heartbeat is 136 / min. The small segment of the fetal head lies in the plane of the inlet into the lesser pelvis. The cervix is smoothed out, its opening is 4 cm. The amniotic sac is absent. What complication occurred during the childbirth?