Low-lying placenta at 18 weeks usually migrates upwards as pregnancy
progresses unless there is some endometrial damage, a scar in the uterus
or placental abnormality. There is a 4-8% chance of recurrence. As you are experiencing similar symptoms as to your first pregnancy, you should observe utmost care. Please take absolute bed rest
, avoid intercourse and get sonograms to observe the status of the pregnancy. As you have been given uterine relaxant, the irritability should wear off. Please follow your doctor's instructions. Take care.