The majority of maternity photos are taken in a standing pose that is between a profile and a 3/4 view. For these poses, more often than not, having your subject place her nearest foot forward will result in a better photo.
The most common way to do this is to have the mom-to-be raise her leg slightly, resting her nearer foot on her toes rather than placing the foot flat on the ground. This small, simple change creates a better line in the photo. It provides an appealing S-curve both in a full length photo, and in a closer crop.
Reverse the pose and suddenly the lines don't work. Raising the far leg looks odd and awkward...
A quick glance should make it clear. Part of the reason this works is that it reduces the amount of visual area the legs, thighs and, not to be indelicate, the butt take up in the frame. Again, by reversing the pose, we get the opposite effect, which better demonstrates what I mean...
While the prevailing ideal of pregnancy is that the mother-to-be will grow only a cute, round belly, this is not the case for 99 out of 100 expectant women. Most pregnant women will also gain weight in the the hips, thighs and butt. While this is normal, most prefer to minimize those changes in their portraits. This trick helps to accomplish that without hiding anything. It simply gives everything a better, cleaner, sleeker shape. Cropped in close you can better see how this works.
Another way to accomplish this effect is to have the nearer foot placed pointing towards the photographer and stretched out slightly. As you can see comparing the two photos below, this can have a very dramatic effect. Perceptually the thigh appears longer and thinner, providing a greater juxtaposition of straight lines vs. the roundness of the belly.
