The graph below portrays a labor curve for a woman, gravida 3, para 2, at 39 weeks' gestation. Membranes were ruptured at 4 cm. The fluid was clear and there has been no indication of fetal distress. Previous infants weighed 3500 g and 3750 g. The estimated weight of this infant, which appears normal on ultrasound, is 3200 g. This labor curve is compatible with which of the following conditions?