There has been no evidence that says that stress is an exact cause of miscarriage however when you're stressed you have much higher chance of having a miscarriage.
So you need to avoid being stressed out and avoid anger and anything else that could raise your blood pressure until the pregnancy is over with.
However women who're stressed during pregnancy have been having miscarriages more than those pregnant woman who were not stressed out.
So stress could actually be a cause of miscarriages but it's not known yet if that stress or anxiety is actually a direct cause of miscarriage or not.