16. A 32-year-old woman, gravida 3, para 2, with type 2 diabetes mellitus is admitted to the hospital in labor at 38 weeks’ gestation. Her first two children were born after normal spontaneous vaginal deliveries. On the admission the cervix is 2 cm dilated, the membranes are intact, and the fundal height is 42 cm. Four hours after admission, the cervix is completely dilated, and the vertex is occiput anterior and at -1 station. Over the next hour, contractions occur every 2 minutes, last 60 seconds and are 60 mm Hg in intensity, but the cervix and stations remain unchanged. Fetal heart tones are normal and reactive. One hour later, her condition remains unchanged. Which of the following is the most likely cause?