Congrats! As an adoptee, I am so excited to hear of an adoptive mother wanting to be involved for birth mom!
Most maternity pants give a size conversion. I have found Motherhood and Maternity run really true, as well as hold up well. Go for pants with a full belly band for her. She can fold it down now, but fold it up, and they will last clear through till the end, plus be nice for postpartum. Anywhere you shop though, you can talk to a sales clerk and ask! I'd also tell you to pick up a cute ruched side top for her if you are in the mood. They are comfy, continue to fit, and help us preggies feel a little stylish with carrying!
This early on, appointments are infrequent anyway. I didn't see my Midwife till I was almost 12 weeks. Talk to the birth mom and see if she had a pregnancy panel run, and also remind her (and offer help if needed) that her records from her previous appointment should be sent to the new office. As for any other screening...
Each office has their own standards, and each woman has her own comfort level. I'd ask you to discuss with the birth mother PRIOR to the appointment HER willingness/desire for any tests that are optional or additional. She obviously wants what's best for the child she is carrying (she chose to make the hard choice and place afterall!!), so I'm sure she's going to have some idea as to what she is/isn't okay with and does/doesn't find necessary for testing.
Some women have virtually none done. My husband and I personally refused anything that wasn't necessary to determine mine or baby's risk during pregnancy. Some women want to know, although there is no knowing for certain until baby is born, if there is some abnormality, and will run every test in the book to make sure. Some women are at high risk for certain things and choose to be tested for them. Some women are just considered low risk and hence opt out. Talk to her first, and then the two of you can talk to the doc together.
Good luck with everything! You have made an amazing decision to adopt, and your birth mother a wonderful and self less decision to place!