The medication probably would not be a good idea in pregnancy. I have not seen any data on the INTERSTIM used during pregnancy either. But you might consider INTERSTIM and turn it off while you are pregnant. My understanding is that autoimmune disorders like MS can sometimes get better during pregnancy, and that may be to your benefit.
Other options include having suprapubic bladder tube placed (invasive and reserved for very severe cases of retention). Botox has also been touted for helping to relax the urethral sphincter, but I have not had good success with it for that purpose. Personally, if you can reliably self cath yourself, and your retention requires it, then self cath can be safe and reliable in the long term. I have some patients who have been doing it for many years without problems.