K.P.
I agree that having a midwife assisted birth is wonderful -- they are less likely to try various interventions than an OB. I also think that having midwives change shifts during your labor is o.k. -- if you've met all the midwives in the practice through the course of your prenatal visits, you should be familiar with all of them anyway.
I delivered baby #2 and baby #3 completely naturally at a birth center in Annapolis -- there are actually two there -- Bay Area Midwifery and Special Beginnings. It was WONDERFUL. I would recommend a birth center to everyone! And for baby #2, the midwife who was with me at the beginning was different than the one with me at the end, but I have no idea when they changed! (LOL -- I was too busy with other things to notice!) The nurse there (a midwife in training) was with me the whole time.
If you really want a natural birth, maybe you could consider a birth center, even though it's not in the Rockville area.