For your existing pregnancy, I suggest you see a doctor. What does the doctor say?
There are herbs which have been used historically to promote miscarriage or abortion. But it is illegal to prescribe them in most developed countries.
There are herbs which may be used to reduce the likelihood of miscarriage in the future, and increase the likelihood of having a healthy baby. Examples include False Unicorn Root, Blue Cohosh and Cramp Bark. It is also important to have a healthy diet and lifestyle.