Pregnancy and Exercise

I've been told by Dr that its ok to exercise,is there any exercises that i should avoid at 21 weeks.
He told me that curves is good but i think the machines are too straineous. 

My OB just told me not to do anything that is high impact or too strenuous.  She suggested walking or the elliptical.  She also suggested using handweights no heavier than 5 - 10 pounds.

My OB / MFM Dr told me that I should avoid anything that would be straineous and the best were very low inpact things like walking and swimming.  He told me the best is swimming to help get that strain off your body but still get the benefits of working out. He suggested DAILY exercise- and told me to keep very hydrated and not overheat.   Oh- he specialized in exercise too.  Great guy- always hated his advise, cause it was a clear message to exercise and keep in shape for the big event and after. Good luck!