Hi Denise -
Are you trying to have children but so far not gotten pregnant? It sounds like the other woman’s being pregnant plays an important role in your dream. If so, you may be worried about your ability to have children and then - the irrational insecure thought/feeling comes - maybe my husband will leave me for another woman who will bear him children. In your dream your husband doesn’t leave you - he just starts up with someone else. This part of the dream is interesting, because I think it represents your security (he doesn’t leave you) and insecurity (the new woman) at the same time.
You also may also be comparing yourself to other women, on days when you are not feeling great about yourself, and deciding that you are less attractive than they. Then you think - why would my husband stay with me? There are all these “better” women out there who are available... In this case, the dream may be reflecting your own feelings of insecurity and self-doubt, rather than being a signal to you that your husband is looking elsewhere for fulfillment.