The itchy rash (if they are not stretch marks) on the abdomen of your pregnant wife could be due to
1. pregnancy itself - this usually resolves spontaneously after childbirth,
2. one of the medicines she might be taking. Sometimes it may even be due
to preservatives, etc in the vitamin preparation, or
3. any other skin problem which can occur in a non-pregnant person.
A Clinical examination is necessary to differentiate between the above possibilities. The line of management would vary accordingly.