A 32-year-old woman in her second pregnancy presents in labor at 42 weeks of gestation. The antenatal period was unremarkable. The baby is estimated to be 3.9 kg in weight, without anomalies. She has regular and strong contractions. However, progression through the second stage of labor is slow. A ventouse is applied. The head of the baby delivers easily, but retracts immediately, with the baby's chin abutting the perineum. Her first pregnancy was uncomplicated, with a 4.0 kg child delivered vaginally. Her medical, surgical, and family histories are unremarkable.