With sore knees and achy back in pregnancy, the most important medicine is to listen to your body. Hot baths, slowing down, and minimal lifting can all help. One of the first things you should do is spend a bit of time with an exercise professional that you trust. Although your change in bodyweight and overall physical structure can be contributing to your pains, there is a chance that you may not be exercising with proper technique or at the best intensity. Getting a thorough evaluation of your posture can also identify any red flags in your alignment. A good trainer can rule out any problems with the way you are currently working out, and design an appropriate program for you, taking into account your exercise history, physical limitations and on-going body changes due to your pregnancy.
Short of this, some simple recommendations would be to spend a bit more time safety stretching the muscles around your pelvis and lower back, and possibly trying to incorporate water exercise like swimming or water aerobics into your routine over the next few months. The buoyancy of the water unloads the joints while you exercise, while the added resistance of the water gives your muscles a good workout. An added benefit is the cooling effect of the water on your body to help prevent overheating.
-Shari Feuz, Mixx Fitness