1) What effect does "stress" have on fertility?
2) Can stress cause miscarriage?

Both questions have the same general answer. From a scientific point-of-view there is no easy, reproducible method to measure "stress," even though everyone knows when they feel it. Thus there is little (if any) data on the effect of conception and miscarriage.

As a general rule, the type of daily stress that people worry about is not known or suspected to interfere with either fertility or successful pregnancy. For perspective, all persons in all walks of life since the moment our ancestors could reason, experienced substantial stress. Though it is beyond the scope of this response, it is not reasonable to believe that modern living with its significant increase in affluence actually resulted in more stress.

Extreme stress is another matter. Though not linked to miscarriage, a certain percentage of women will stop menstruating as a result of psychic distress such as divorce, incarceration, or the death of a loved one. Stresses less severe have not been linked to reduced reproductive function.

It is worth noting that the miscarriage rate for any given pregnancy is on the order of 15-20%. A history of one miscarriage does not substantially increase the risk of miscarriage on the next pregnancy. On a personal note, my wife lost our first two pregnancies even though she carried the next four in sequence.

All women who miscarry are more anxious with the subsequent pregnancy. The only thing one can do is to accept the anxiety as a natural occurrence and do the best one can to cope with it. The odds remain in your favor.