Doesn't hurt to ask. I had a doctor that wouldn't do mine until over 24 weeks. I switched doctors over it. I don't see why a doctor would wait until 30 weeks considering the baby starts to run out of room and it can become harder to see everything. Call and ask, or wait until your next apt to ask!
Yeah I would call and demand to have it changed to the 20 weeks or I would be finding another dr ASAP.
It was my mistake. I rang them up (the hospital where I had my scan - they gave me the appointment) and the receptionist said that the 30 week appointment is to see a doctor (along with the dietician I am to see soon) because my weight is so low and that I'd get my 20 week appointment in a letter through the post. I apologised a lot but she said it was fine and to call if I don't get my appointment or I need anything else. She was so nice!