My doctor uses a Doppler for every appointment to hear the heartbeat. I had an ultrasound at 36 weeks, but they told me when they use the Doppler that if they hear the heartbeat with the Doppler positioned BELOW your belly button, then baby is head down, if it is ABOVE, the baby is breech.
My doctor wouldn't give me another one after the 20 week. He said as long as the baby is head down, and there haven't been any complications, then there is no real reason to do another. He pushes down really hard on my stomach and can feel that the head is down. If your baby is breech, they would be able to tell and would not have you naturally deliver that way. I was bummed that I didn't get another sonogram, but I'm glad that its because he and I are both healthy.
I has an ultrasound yesterday to see if my girl is head down. I wasn't supposed to get one but the doctor couldn't tell her position. My doctor had originally said that since my 20 week sonogram looked good, I didn't need another.