Portrait photographer here. When I set up maternity sessions, it's usually around the 32-34 week mark. I find that's the best time for the mom (not too tired/big to get around easily) and the bump looks well-rounded by then as well.

Don't worry so much about poses if your photographer is experienced. Let him or her guide you (they should be able to provide succinct instructions) as they have done this many times before. Bring a few ideas along if you have something specific of which you'd like (pose/prop) but you should be able to rely on the photographer's knowledge of what looks good from behind the camera.

That said, chose a photographer based on their portfolio. Look through their work to see if you like the style, posing methods; talk to them about their experience prior to booking and your comfort level with them. Maternity photography can be an intimate session (possibly with full nudity, depending on your comfort level) so you want to make sure you're comfortable with the person behind the lens.

If you're getting photos done around your house (or someone else's home), try to ensure the areas being photographed are as decluttered as possible. Clutter makes for distracting and messy photos.

Dogs are great for photos (they truly understand candid photography!) but I would also have someone who is not part of the photos standing by for the parts of the session where you might want some images of just you and SO so they can doggy-sit.

You should also be able to ask your photographer for instructions (what to wear, props, etc.) on how to prep once you've booked them for a session and they should be able to guide you in the right direction.

Finally, don't stress too much beforehand, trust your photographer, and have fun with it. Spend time laughing and smiling with love during this fantastic moment in your life and enjoy the process of having that captured for you to remember in years to come.