Materinity photography is portraiture so the key goal is to show the person. That means they should be prominent in the frame. The location is also important. I look at this image, and I think, "Why is there a woman in a long dress standing barefoot on a gravel road?". It doesn't make sense to me. Additionally, the road is a great leading line, but your placement of the subject, her clothing choice and your use of the vignette mean that the eye follows the brighter road right past the subject and into the middle of the frame. If you'd had her further back and in the middle, that aspect would have worked well.