I agree with Helene Byrne in her answer above. Make sure they don’t do any crunches or anything in a crunch-like position. Nothing in a quadraped position where the internal organs would be pushing out and causing pressure on the linea alba and pushing the recti apart. Watch alignment carefully and don’t let the ribcage splay out.
All pregnant women should strengthen their deepest abdominal muscle, their Transverse Abdominis, or TvA, to prevent common pregnancy related complaints and complications such as diastasis recti (abdominal separation), back pain, and pelvic instability. As an added benefit, because the TvA is the primary expulsion muscle, maintaining strength in this muscle greatly aids in the pushing phase of labor.