It's okay to call an ambulance. My husband was a volunteer EMT for a long time, and he always emphasized to me that emergency workers are mostly sitting around waiting for you to call them, so you should never hesitate or feel guilty about doing so.
Plus, they are trained in emergency delivery, so they will be able to handle worst case scenario.
Don't panic firstly; call your doctor and see if they can sort something out for you, if not call the ambulance once it's time.
Just have faith that God will work it out and believe that he will....Everything will be ok