Pregnancy changes you, suddenly you are responsible not only for yourself but for another life that is growing so trustingly inside you. Everything you do suddenly becomes a lot less selfish and holistic.
One of the best exercises during pregnancy is the standing yoga pose. Of course you should do the most basic ones and not those that require you to balance on one leg or something equally impossible. Remember that you are beginning to become like a heavy ship now, and it is best to avoid things that could lead to accidents. These exercises will strengthen your legs and muscles and help with circulation while reducing leg cramps too.
Some amount of moderate twisting and bends is allowed and even recommended. Your spine will become strong and flexible; posture will improve and so will blood circulation. The improved muscle tone of spine, abdomen and pelvis helps to support the added weight of uterus, prevent backache, leg cramps, breathlessness, feet edema, etc. In the long run it will aid in child birth and post partum recovery.
Pranayama or the breathing exercises, and meditation also improves the nervous system, emotional stability, and even helps a woman handle anxiety, fears and phobias. It also can make better pregnancy related problems like insomnia, high blood pressure, and breathlessness. The metabolism and oxygenation improves, giving you an inner calm that is good for both the baby and you. The best thing is that you will learn how to control your breathing during labor, making the entire process so much easier.
Do not try complicated exercises, or those that require you to lie on you stomach. You must not be on your back for a long time, because it will decrease the blood flow of the dorsal vein and even add to back pain.