It wouldn't be proper to give any medical advice regarding the safety for any future event. But mitral valve stenosis may not place any limitations depending on the degree of stenosis. For some insight, serious MVS compromises the amount of blood pumped into circulation with each stroke. The deficiency of blood pumped will cause the heart to work harder to meet the blood/oxygen demand. A harder working heart without treatment can stress the heart to begin to fail. Minor stenosis may not be a problem. I don't believe MVS would cause a miscarriage..your wife's health would probably be the issue.
If MVS is serious, it can be repaired or replaced.