In this pose, the mom-to-be cradles her belly with her hands counterpoint to each other — one hand up, the other down. The leg closer to the viewer should be a bit forward and may be raised up. A slight turn in the upper torso helps this pose to engage the viewer.
A clear placement of the hands is important. Note that the first portrait on the right is more sucessful because the placement of hands is clear. In the second photo, the fingers are harder to see and create a visual ambiguity. It feels off balance because the mom's left hand is out of view and we don't have any clues to let us know where it is.
With this pose, you have several options based on which hand is forward and where the hands are located. I recommend having the mom-to-be move her hands around and try to feel the baby and the shape of her belly. Starting with the cradled pose, you can experiment with the placement of hands. This will give you more options when you are editing and tends to ellicit a more genuine and positive expression. Having the nearer hand at the top of the belly tends to work better.
It is also worth experimenting with the overall position of the hands, with the mom feeling her belly. Notice how different the effect is when your subject reverses her hands, brings them together, or looks down at her belly. You don't need to limit yourself to a strict, one hand up, one hand down counterpoint pose. Just keep in mind that if one hand isn't visible, the pose falls out of balance.
There is also a little trick to this pose. The placement of hands, which are easy to guide, and very low pressure, will also direct your subject's arms, which can be used to help minimize or hide any "back fat." While this is a normal part of pregnancy, most moms-to-be don't want it commemorated. Everyone is different, but this pose gives you an option to make a more flattering photo without saying the words "back fat" out loud - a phrase I recommend avoiding. When you are directing the pose, you may simply ask the mom-to-be to move her arm a bit forward or back in order to make the pose more flattering.
Finally, keep in mind the tip about keeping the best foot forward. It would seem that a pose that is most about hands, belly and expression might be exempt, but compare the three photos below and see what a difference the position of legs makes.