Check with your doctor, but in most cases it's safe to continue with your workout AS LONG AS IT FEELS OK. If it doesn't feel comfortable, don't do it! If you've never done it before, now is not the time ot start.
I have always been a walker/runner. During my pregnancy, I continued power walking, but stopped running when my belly started to show b/c I felt unbalanced and achy after doing so. I also continued with very light weight lifting (think 5 pound hand weights), but nothing more than that. I did prenatal yoga because I had done yoga prior to becoming pregnant.
Here's the thing my doctor did tell me (aside from the above)... if you're tired, sleep. If you're hungry, have a snack. If you feel like you need to take a walk, take one. Don't force yourself to exercise on days that you are too tired. If it feels "off", stop doing it.
My girlfriend is an avid gym rat and is about 15 weeks pregnant right now. We were talking the other day b/c she has been doing her regular crunches and abdominal workout right up until this week. She stopped because it just felt "off". She listened to what her body was trying to tell her and tagged on 10 extra minutes of walking instead.
Some days I power-walked for 40 minutes because I felt great... other days I walked at a moderate pace for 30 because that's what felt OK.