Because maternity portraits are profound and capture a time of momentous change, they represent a fantastic opportunity for a photographer.
A popular misconception about maternity photography is that it is all about photographing the pregnant belly. This is not true. Approaching the subject this way leads to photos that are anonymous, dull and, in the worse cases, fetishistic. (see Just the Belly) The pregnant belly is an obvious and outward sign of impending change and new life, but the belly cannot be the sole subject, even if it is all we see within the borders of the photograph.
While the pregnant belly should not be the sole subject of a maternity photo, it will be the focal point for most maternity photos. The great thing about maternity photography is you always know what your subject is!
If you are feeling tentative about what to photograph, try focusing on the belly and the mom's expression. If these two things fill your frame, you are more than halfway there.
Maternity portraits should be about the person. They should be about the mother-to-be's relationship with the world around her, with her baby, her changing body, her changing identity (or retaining her identity), her family, her place in the world and her sexuality. Maternity portraits should be about the moment, the waiting, the ephemeral time that will soon be gone, the joy, the hopes and the dreams of the mother-to-be.
(Not all at once, of course, you'll hurt yourself.)
A small, subset of maternity portraits take a more formal approch which is neither about the belly itself or the expectant mother, but which is concerned with treating the body as landscape or abstraction. There is a long tradition of this kind of photography, most of it in the form of art nudes. These photographs tend to experiment with line, structure and composition. To work well, these photos require a strong sense of composition and form. As a result, I do reccomend this approach for begining photographers. (For more on this subject, visit the section on line and form.)
That should not discourage you from attempting to shoot something with artistic value. Your job as the photographer is to capture something meaningful. Otherwise, you are only taking snapshots.
It is also possible to capture the negative aspects of pregnancy, the discomfort and sacrifice, the nervousness that can accompany new parenthood, but this guide will focus primarily on the positive.