A 24-year-old G2P1 at 28 weeks of gestation presents with regular uterine contractions (4 in 10 minutes) and mild vaginal spotting. She denies rupture of membranes. Vital signs are normal. Speculum examination shows a closed cervix with no active bleeding. Fetal heart rate is 140 bpm. Ultrasound confirms a singleton pregnancy with appropriate growth and amniotic fluid volume. Cervical length by transvaginal ultrasound is 2.8 cm. What is the most appropriate next step in management?