Now your question is clear. There should not be any reentry problems when you try reenter only with PP's I551 stamping. There are some experience threads poste before here on that issue.
Entering while pregnant should not be a issue as pregnant women even in H1/H4 visas are allowed to enter. So entry while preganant with PP stamping only should not be an issue at all. It seems there may be some minor irritants or secondary inspection at POE while using the PP stamping to enter. Immigration clerks in some Airports may not be aware of or may not recognize this tiny obscure stampings as a equvivalent of greencard. In that case we need to educate them with the help of lawer's letter, approval notice, information from BCIS web site etc. In the worst case, we may have to ask them to contact the local US consulate to cross check and get the clarification.
Hope we will not have any such issues. I am also planning to travel with PP stamping only by Jul 3rd.
Best Wishes