Maternity Photoshoot in Jeans?

So my stepsis' BF is a pro photographer and in town this weekend. He offered to take maternity pics for free which is so nice of him. I have nothing to wear and feel like I should wear a dress but I'm too tired to go to the mall. I don't feel good in anything right now besides leggings. Would it be weird to wear jeans or leggings in a maternity shoot? I feel like it might be too casual... I also have my baby shower dress but I already have lots of pics in that one. Or should I just force myself to go to the mall... Ugh.