You're right that there is some self-selecting going on, but you do what you can with it.

As to your point about home birthers not getting ultasounds within the first 20 weeks----I don't think ultrasounds are needed within 20 weeks right? That was my understanding, unless I'm wrong. I know doctors often do them earlier because they have the machines in their office, and insurance is paying, so why not? But I'm not sure there is really a medical need --- in MOST pregnancies---to do it.

That being said, another reason probably at play here is that home birthers do tend to be the more extreme group in the natural birthing movement, and tend to be the folks that don't want medical intervention at all. There are many natural birthers who aren't that extreme and still have their babies in hospitals.