A full nude presents a significant challange if your model wishes to remain covered. Because she only has two arms and hands to cover with. This makes your range of options is limited.
For a full, standing maternity nude, the "Demi Moore" pose, made famous by Annie Lebovitz is usually your best option. This does not mean you need to replicate the photo. What is helpful is to understand why this is an elegant solution to a somewhat complicated problem.
The first consideration with a full nude is the placement of hands. Most moms-to-be will almost instinctively take this pose to cover up. In order to keep everything discrete, one hand needs to do all the work up top so the other will be free to cover down below.
The three photos above provide a fairly clear view of this pose. In the first image, the pose itself is a little static and the hand is not entirely covering as it should. The second image corrects the problems in the first, by bringing the model's left leg up. The pose now works, though it could be improved. Because of the angle of her leg, which is dipped in slightly, the pose is less flattering to her thigh and butt. In the the third image her leg is turned out slightly, which thins her rear and thighs. Her improved posture and more engaged expression help as well. The final image above brings her arm back, and connects her gaze with her baby, which is a nice variation on the third image above.
This pose can be varied somewhat, but there is only so much latitude you have with the placement of hands and position of the body before your subject is no longer covered and you have a full nude.
Using this pose does not mean you need to slavishly recreate the original. Even when attempting to do so for this guide, I found a wide range of subtle variation that required many, many attempts to create the opening image above.