Probably depends on what stage her pregnancy is at. Morrigan transforms into a dog after the dark ritual is done, to hang out near Sten in the post-game celebration at the Ferelden imperial palace. Most players don't realize it's her, but if you tell her to sod off instead of doing the ritual she turns into the same dog seen at the celebration. Guess she didn't want to miss the celebration, even if she was subtle about her presence and didn't want anyone to know it was her.
Her after sodding off-

Out of place dirty dog during the celebration that looks pretty much exactly her dog form in the above sceenshot-

She can still transform in gameplay too, not that it counts much thanks to gameplay and story segregation. Then again, that is after about a day or 2, when her "child" is still about 8 individual cells floating around in her uterus. If she did it any later in the pregnancy, perhaps Kieran would have had dog ears and a tail
.