A more flexible and less revealing pose is to drape one arm across the breasts, so that the forearm is providing most of the coverage. (See left.) It is a protective gesture. As a result, it tends to be a more comfortable place to begin. It also frees up one hand so the mom-to-be may place it on her belly, creating a variation on the cradled belly pose.
This method provides a good amount of coverage, though it will take a bit of work to get the angle of the arm right. You want to make sure the far hand is providing the coverage it should. This can take some time, especially if the mom-to-be is well-endowed, because one breast or the other may get smooshed in place. It is important to try to keep placement as relaxed and natural as possible.
To keep your photographs tasteful and discreet, the mom-to-be should be covered from your point of view. What matters is what shows on camera and, specifically, in the final photograph. This may mean extra work, but it also gives you and your subject a greater range of options.
In the photos above, the model's left arm and hand provide coverage. The clearer placement of the model's right hand makes the pose work much better. It is very subtle, but if you look closely at her right shoulder, you will find it is more relaxed in the third image, which improves the overall feel of the pose.
Moving the arm higher allows the forearm, rather than the hand, to cover the far breast.
Looking directly into the camera can be difficult, especially with nude or semi-nude materntiy photography, because few women are 100% comfortable with posing nude. Having your mom-to-be look down, away or close her eyes is often a better, choice. This is especially true if you are working with low key lighting, where the subject's face is in shadow, and her gaze would not be seen.
Another method that can work is having the mom-to-be place her hands in a "sleep position." In the series above, the first photo does not look quite right. While it provides coverage, pointing the hands straight upwards tends give the appearance of prayer, which is usually not a good mix with nudity. If the mom-to-be simply tucks her hands back, it reads much better. Closed eyes and clasping of hands can work as well, as shown in the final three photos above.