As her pregnancy progresses, an expectant mother's center of gravity shifts. The changes to her body make maintaining good posture difficult. There is a tendency for pregnant women to slouch, especially in the later stages or pregnancy.
While this is understandable, it does not make for a dynamic photo. Fortunately, the remedy for this is simple: Remind your subject to stand up straight and put her shoulders back. In fact, a slight arching of the back can help improve the dynamic qualities of a photo.
The important thing is for you, as the photographer, to be aware of your subject's posture throughout your session.
Pregnancy is a huge change and maintaining good posture is difficult. You will want to gently remind your subject to correct her posture if you notice it is coming out of line. You may take the opportunity to have your mom-to-be stretch from time to time during the session. It should feel good and will help her better transition to good posture.
While you do want to present your subject as naturally as possible, where posture is concerned are better off with an exaggerated arch in the back, than a natural slouch.