From what I can tell, CPMs are still being taught this because their certification (which comes through the Midwifery Association of North America) doesn't route past universities so much or, you know, modern basic pregnancy science. The sheer biological implausibility of this claim has become apparent in the past decade, but their sources are older than that.
I would need to use the Wayback Machine to pull their recommended sources, as they're not listing them on the webpage for North American Registry of Midwives any longer, but most of them were quite out of date the last I checked. Anne Frye, Ina Mae Gaskin, Helen Varney, etc. A few years ago I did do this, pulled all the texts and read the sections on preeclampsia. Every last one of them explained preeclampsia using Brewer's mechanism. Cranky writing followed; I tried to get a few professional magazines to pick it up, but they told me that it "wasn't of interest to a sufficient number of readers." Because only one in twenty women gets this, and they don't have families?
There's some pushback within the community itself, which is promising: http://navelgazingmidwife.squarespace.c ... amned.html
and http://birthunplugged.blogspot.com/2011 ... s-not.html