Back pain or prolonged active labor is sometimes caused by a subtle malposition called asynclitism, in which the baby’s head is tilted and the top of it isn’t centered on the cervix.    From Wordnik.com.  [Pregnancy, Childbirth, and the Newborn] Reference

Clues that suggest asynclitism include delayed dilation or uneven dilation, in which most of the cervix dilates completely while the remainder doesn’t, creating a “cervical lip”.    From Wordnik.com.  [Pregnancy, Childbirth, and the Newborn] Reference

With asynclitism, the head doesn’t press evenly against the cervix which makes dilation less efficient and it doesn’t fit through the pelvis as well as when it’s centered on the cervix.    From Wordnik.com.  [Pregnancy, Childbirth, and the Newborn] Reference