Diastasis recti is a partial or complete separation between the left and right sides of the rectus abdominis muscle. It is commonly seen in women who have multiple pregnancies, because the muscles have been stretched many times. Three main factors contribute to the incidence and severity of diastasis recti during pregnancy: maternal hormones (relaxin, estrogen, and progesterone), mechanical stress within the abdominal cavity due to increasing girth, and weak abdominal muscles (strong abdominal muscles are more likely to resist this condition). While some rectus abdominis separation is a normal part of every pregnancy, too much separation may lead to diminished muscular force production and even more separation during physical exertion. The most common test for diastasis recti is performed by placing two fingers horizontally on the suspected location while the client lies supine with the knees bent and performs a curl-up. If the fingers can penetrate at the location, there is probably a split. The degree of separation is measured according to the number of fingerwidths of the split. One to two fingerwidths is considered normal, whereas greater than three fingerwidths is excessive and care should be taken to avoid placing a direct line of stress on the area.
Abdominal compression exercises and curl-ups in a semirecumbent position may be helpful for strengthening the rectus abdominis in this situation. Movements that involve subtle contractions of the deep abdominals (i.e., pulling the belly button in toward the spine) are best for women with this condition. Stabilization exercises on both hands and knees (quadruped position), such as the cat/camel and birddog are good choices. Pelvic tilts while lying with the back and feet on the floor with the knees bent will help engage the lower abs and transverse abdominis. Also in this lying position, leg slides can be performed where the lower back is held to the floor using abdominal stabilization while one heel at a time slides out and back in.
Women with diastasis recti should avoid any exercises that put extra stress on the abs or cause them to stretch, including twisting exercises and hyperextension of the back (e.g. cobra stretch or arching over an exercise ball), as they may cause further separation. As you already mentioned in your question, traditional crunches should be avoided as they cause abs to bulge out rather than compressing them inward. Lastly, heavy lifting, straining or intense coughing without support that can cause you to press outward with the abs should be avoided until the muscles are strong enough to resume their original anatomical position. In some cases, diastasis recti requires surgery for full recovery, so these exercises should be attempted only after a woman has been evaluated and cleared by her physician for these types of exercises.