Tell her to rub her belly and talk to her baby. If she feels awkward doing this, just have her smile while looking down at her bump. Have her think of meeting the baby. This will get more authentic expressions in the portraits.
Take full length, mid-length, and up-close photos of her. Once you feel like you have taken enough solid portraits, experiment with different crops.
Asking for her to bend one leg can not only help to relax and shift the weight, but it also adds more interesting lines leading to the belly, which is the focal point of the whole session.
During her portrait session try and keep her hands moving as well as where she is looking. Have her look at you, at her partner, down her shoulder, at her belly.
Huge tip: When she looks at her belly, have her poke her neck out towards the camera and actually look at her feet. This will avoid getting any squished chins.
Dad With Belly
Just because the belly is the focal point doesn’t mean dad can’t have his time in front of the camera. Have dad kneel and get close to the belly with both hands on each side. Have him talk to the belly and enjoy the closeness.