I think more of the models back should have been included to add contours to the photo. Perhaps composed to include the lower back and the rise of the buttocks. Also, watch for underwear marks.
The image is a little flat and uninteresting. Also the recent removal of a bra shows the subject has just gone to bed. Before nude photography at east 15 minutes to 1/2 an hour without tight fitting clothing. A face would have dramatically improved this image.
love the idea of this one. might need a bit of contrast, and you might wanna wait an hour or so to avoid the lines of her bra being visible in her back :)