I often get asked what to wear to a maternity session. This is a person decision and its up to your comfort level.
Some woman want to show their body off and wear nothing and some want to wear their husbands botton up shirt and a pair of jeans. Whatever you choose just make sure there are no big prints or logos and be authentic to your style.
I usually suggest bringing a bra and underwear set, a pair of non maternity jeans and then come and pick from my selection of dresses in my client wardrobe.
Typically I do maternity photos around 31-34 weeks and they can be done in studio or outside.