A 28-year-old primigravida at 39 weeks gestation presents to the labor ward with regular uterine contractions (3 in 10 minutes, each lasting 40 seconds) and vaginal bleeding. On examination, the cervix is 3 cm dilated, 80% effaced, and the fetal heart rate is 140 bpm. She reports rupture of membranes 6 hours ago with clear fluid. To confirm entry into active phase of labor and assess fetal well-being, which is the investigation of choice?