This pose was specifically chosen for this mom-to-be who was carrying her pregnancy very low. By leaning back, her belly is raised up a little. The use of the shirt puts the focus back on the mom-to-be and her silhouette than how low her pregnancy is resting.
To compare, below is a photo where no such consideration was given, that simply demonstrates how low she is carrying:
Interesting, perhaps, but not a flattering or artful photo.
These photos were taken for the 2nd edition of my Pregnancy Photography Guide, which discusses includes lots of pregnancy posing ideas, (including this one.)
Still available for iPad