Yoga can especially help this issue when you learn to take longer breaths and to practice certain postures. Examples of these techniques include doing full breath and practicing postures such as child's pose with the knees wider to accommodate the stomach. You can also lay on your side and put a pillow between the legs, which will provide relief from the heavy stomach.
What postures you do depends on what is causing the discomfort. Is the baby moving a lot? Is it the size of your stomach in proportion to the body? Is gas getting trapped in your GI tract? Is the size of your stomach putting pressure on the lower back?
Once you know what the source of discomfort is, then you can begin to experience relief through different postures. The full breath, however, is a wonderful tool for mental and physical relief of the whole system, and meditation is also a very valuable tool for resolving mental fatigue and processing strong emotions.