In the third trimester, follow the goals mapped out in the first two trimesters— add avoiding inversions to the list (because of potential falls). And, really make an effort not to hyperextend joints or to stretch them to their full ROM (range of motion). Be gentle if you are using free weights—make sure that you do not lift weights overhead or over your belly, simply to avoid the potential of dropping the weights on yourself. This may sound silly, but with your new center of gravity, loose joints, and other pregnancy changes, balance and strength become a real concern.
During this trimester, many moms report slowing down. Their bodies have made substantial changes, including increased hormones, higher caloric needs, and new sleeping habits and positions, to name a few. Also, their center of gravity is different, their frame is carrying substantially more weight, and they may have swelling in the lower legs, ankles, and feet. On top of all that, the joints are loose, and therefore stability is lessened; the body must learn to move differently than it did pre-pregnancy. These changes, and others in the body suggest taking a slower approach.
Exercise in the third trimester usually points to calm, relaxing practices. Many expecting moms like to focus on breathing and mental strength to prepare for labor and delivery. Some popular choices include walking, yoga, Pilates (however, not focusing on the abs), and swimming. Even though you may feel completely different from your usually energetic self, embrace this time. Enlist naps, walks, massage, time for reflection, and anything that lets you indulge, feel comfortable, and enjoy the miracle of which you are a part.