To begin understanding of fetal positioning, it is helpful to understand the basics of back labor  and the terminology commonly used when assessing fetal positioning.

The terminology used by medical professionals is loosely based on the five boney plates that make up the fetal head (cephalis) and the position of the anterior and posterior fontanels or soft spots felt during a vaginal exam.  Early in labor it may be difficult to feel the anterior fontanel because the baby will have flexed the neck and tucked the chin so the smallest portion of the head enters the pelvic inlet.

Cephalic Positions

OA or Occiput Anterior indicates that the baby is coming down the birth canal, head down, with the posterior of the head coming under the pelvic bone and the baby’s face facing the mother’s sacrum.  Some babys’ heads may be either slightly ROA (right occiput anterior) or LOA (left occiput anterior) or their entire body may be slightly ascynclitic (the entire body may be slightly right or left leaning).

Left occiput transverse position (LOT) and Right occiput transverse (ROT) are common in early labor.  During labor the baby’s head descends with the occiput usually rotating anteriorly, converting this LOT to an LOA or OA as the head is born. If the head fails to rotate despite steady descent, this is called a "deep transverse arrest," and is somewhat common among CPD (cephalopelvic disproportion) babies, mothers with flat pelvises that favor a transverse delivery and mothers who received epidurals prior to 4 cms and had minimal position changes during labor to assist the baby completing the Cardinal Movements.

Occiput posterior positions, including direct OP, LOP (Left Occiput Posterior) and ROP (Right Occiput Posterior) are positions favored by certain shapes of the pelvic inlet and outlet. If the head is at 0 Station, the biparietal diameter (or measurement from baby’s ear tip across the head to the opposite ear tip) is at the pelvic inlet and the head is fully engaged. In posterior positions, at 0 Station, the biparietal diameter is still a couple centimeters above the pelvic inlet, meaning that the head is not fully engaged. Babies can deliver in the posterior position, but the pelvis needs to be large enough and it usually takes longer.

Forceps are often used to deliver babies in this position, but there is controversy whether the fetus should be delivered in the posterior position, or rotated with the forceps to the anterior position. Outcomes with assisted deliveries depend on  clinical circumstances and the preferences of the careprovider.  However it is important to note that posterior presentations carry a higher incidence of assisted delivery, cesarean birth, anal sphincter injury, augmented labor and prolonged labors.

Breech Positions
The terminology used for breech presentations are basically the same as for cephalic positions, except the sacrum of the fetus is used as the primary landmark, instead of the head.

Sacrum Anterior (SA) means the fetal sacrum is closest to the mother's symphysis pubis or pubic bone. Left Sacrum Anterior (LSA) means the fetal sacrum is closest to the mother's symphysis pubis and rotated slightly to the mother's left (clockwise from direct SA). Right Sacrum Anterior (RSA) means the fetal sacrum is closest to the mother's symphysis pubis and rotated slightly to the mother's right (counterclockwise from direct SA). Right Sacrum Transverse (RST)  and Left Sacrum Transverse (LST) indicate a transverse presentation. Right Sacrum Posterior (RSP), Left Sacrum Posterior (LSP), and Sacrum Posterior (SP) indicate the fetal sacrum is farthest away from the mother’s symphysis pubis.

References:

Calais-Germain, B. The Female Pelvis: Anatomy & Exercises.

Fitzpatrick, M., et al. “Influence of persistent occiput posterior positioning on delivery outcomes.” Obstetrics & Gynecology. 98(6), Dec 2001, pp 1027 – 1031.

Ponkey, S., et al. “Persistent fetal occiput posterior position: obstetric outcomes.” Obstetrics & Gynecology. 101(5), May 2003, pp 915-920.

Scott, P. and Jean Sutton. Understanding Optimal Foetal Positioning.

Shermer RH, Raines DA. Positioning during the second stage of labor: moving back to basics. J Obstet Gynecol Neonatal Nurs. 1997 Nov-Dec;26(6):727-34.