What exercises I can do in pregnancy to keep my back fit?

Core strengthening. Your pregnancy will take a toll on your core muscles. Studies have shown that fit moms-to-be have an easier time in labor. I recommend that you 1st talk with your OB about exercise but if safe, I recommend a regimen of back, abdominal, and external/internal oblique strengthening-- planks, abs, etc. Don't overdo it-- I highly recommend that you get a good pt or trainer to guide you.