That’s a hard question to answer from a real-world perspective.
Obviously, there isn’t a pregnant woman on the planet that isn’t under stress.
While doctors say that stress can contribute to miscarriage, they don’t really know why. They think that the release of one hormone (CRH, I think) contributes… but they also think that it might be CRH affecting these mast cells, which in response release histamines and various other chemicals, and that is what contributes to early labor, low birth weight, and miscarriage.
There have also been some studies that didn’t correlate stress with miscarriage at all. One stated that women under a lot of stress are more likely to smoke or use drugs, and that could be a contributor to miscarriage. Some also say that depression is a big contributor for multiple miscarriages.
Of course, it’s a good thing to reduce stress in any way you can. Cut back on caffeine, do some yoga every day, take a warm bath… none of that is negative.