We actual just attended a home VBAC yesterday with a posterior cervix. Baby was in an 'ideal' position of LOA and very low, and her cervix was SO far posterior I couldn't even reach it (the other midwife could just barely). Mom remained about 4-5 cm until the cervix finally came midline, and then labor progressed normally. We don't check women routinely prenatally (or during labor, for that matter), so who know how often a cervix is posterior and we didn't even know it.
Although we did try to bring it forward, it kept going right back posterior,a nd I think what finally did the trick was mom spending time in knees/chest, since her baby was so low.
But, as far as posterior cervix before labor starts? Totally normal, and not a big deal, imo. Early labor contractions (or pre-labor stuff days before labor) will bring the cervix forward almost always. I only share the above birth story as an example of a anterior, non-asyclintic baby with a posterior cervix.
Mama to ds#1 (7) and a ds#2 (1 1/2)