A good place to start is by using the breath to relax the body. Find a comfortable seated position, or lay on your left side and close your eyes. Mentally scan your body, starting with the top of your head and finishing with your toes. As you exhale, release any tension you are holding, paying special attention to your brow and jaw, allow your shoulders to drop away from your ears, and relax your pelvic floor. Use this time to deepen your inhales and lengthen your exhales, allowing your body to follow your breath, becoming soft and pliable.
In your yoga practice, notice when a particular asana becomes challenging. Practice softening into the pose with your breath. Notice how your body changes. This is especially great during Warrior II and other active poses. Let your breath give you a sense of buoyancy, rather than strain.
During poses that work on flexibility, practice sending your breath to different parts of your body to release tension. For example, in Trikonasana or Triangle, send your breath to your side body to allow your muscles to stretch and open, really softening into the stretch. During a contraction, this same skill helps your pelvis relax and cervix dilate, helping you labor effectively and making way for your baby.
Keep in mind, that during pregnancy it is especially important to honor your intuition and follow practices that feel right for you. Namaste!