A healthy 30-year-old twins pregnant woman, G2P1 at 31 weeks’gestation present to obstetric ward, complaining of intermittent abdominal pain. She denies leakage of fluid or bleeding per vagina. Her antenatal history has been unremarkable. On examination, the fetal heart rate is 142/min ,Uterine contraction are occurring every 3 to 5 min. On vaginal examination, her cervix is 1cm dilated, 90% effaced.What is the most likely initial treatment EXCEPT ?（ ）