If you want to maintain your weight on SW whilst pregnant, you just need to stick to the plan (meals and snacks should be mostly free foods) and have a few more healthy extra Bs than usual - this is how target members maintain their weight once they reach target.
However, it's fine to continue following SW while pregnant too (and continue losing weight) - you just need to have a chat to your midwife/doctor about it. When you're pregnant, you only need about 250-300 extra calories per day for your body (and even less than that at the beginning of your pregnancy) so you don't need to change much about your eating habits at all - just stay healthy, eat a well balanced and nutritious diet full of vitamins and minerals and don't forget the folic acid!