I would not suggest going for one of the larger ships. There can be a great deal of walking involved just getting around those ships. A smaller one might be easier for you to handle.
Also, do check with the cruise line re. your pregnancy. I would hate to see you fly somewhere. then be denied boarding. And, having lived in L.A. for a long time, the Maxican riviera is pretty much all that is available out there. Just not many islands off that coast. What about Hawaii?
Hope you find what you are looking for!