I have been dealing with panic attacks since I was a teenager and they recently have come back during pregnancy. When you are in the middle of one, you should breathe deep in through your nose and out of your mouth, slowly. Keep doing it and it will make your heart rate slow down and the anxiety will eventually pass. It is scary and it stinks a lot. I used to be on xanax, which was a lifesaver, but can't while pregnant. Having to go through them without meds is definitely hard, but I just try to breathe and tell myself I need to calm down for the baby. Hope that helps!
Practice meditation, get daily exercise, make a point of listing out loud or on paper if necessary all the positives. This will help to release seratonin and endorphoins. Anxiety can have many causes, usually it is because subconsciously we are uncertain or fearful about something. Bring fears out in the open and don't be afraid to ask for help. Good luck.