Is it CPD or not?

Head size is less important when it comes to fitting through the pelvis than is the angle of the baby's head. A posterior baby will present a bigger head circumference and can sometimes get stuck in a pelvis that the same baby could have fit through if anterior. See what to do to reduce incorrect diagnosis of CephaloPelvic Disproportion.

Many parents think that as long as the baby is head down in the womb the baby is ready for birth. Head down is only half the story.

It is a big relief to find out the baby is not breech (buttocks coming through the pelvis before the head is born). Especially when so many breech babies are born by cesarean these days and so few doctors and midwives understand spontaneous breech birth.  But being head down is only the first step. For the best chance at a natural birth there is more preparation. Baby should have his or her chin tucked by 38 weeks or so. For a first time mother, the baby is expected to engage in the pelvic brim (dip into the pelvis a bit) by 38 weeks. A second baby or more, may wait above the brim until labor begins, or even gets rolling. Then, if we can help the baby's back get to the the mother's left with baby's feet to the right side (only).  Otherwise, labor often rotates the baby into a better position when women use active birthing techniques.

Asynclitism means asymmetrical and is the term used when a baby’s head is tipped towards one shoulder.
Early in labor the baby’s head enters the brim of the pelvis in asynclitism –tipped- to get around the protruding base of the spine (sacral promontory).
When the nurse checks the cervix at 3 cm she’ll notice that the baby’s head is closer to the mother’s front (usually). There is space between the baby’s head and the mother’s sacrum in back.
Normally, the head has usually filled in the space evenly by 4-5 cm. The head has become symmetrical inside the pelvic canal. The head is synclitic.

Asynclitism only becomes a problem when it persists beyond early labor. The asynclitic head has a harder time passing through the narrow part of the pelvis; the ishial spines. Labor becomes longer, and this added length continues through 2nd stage.

In this article we'll discuss what a posterior fetal position is and why it matters in labor. 

The posterior baby, or occiput posterior fetal position, is when the back of baby's head is towards the mother's back. 

The posterior head seems larger than the anterior head. This is because the posterior presentation aims more of the head into the pelvis at once. The head comes in like an oblong, rather than a circle. The top of baby's head comes into the pelvis first.

Here is a brief description of anatomy as it pertains to fetal position.
We’ll go over the mother’s anatomy and then the baby’s.  To see how they work together during birth, see The Birth Process in the section on Birth.

What is "the most practical way of making sure my baby stays anterior? I've had TWO posterior babies! UGH!”

Mainly, the baby will "stay" anterior if your womb is balanced (symmetrical, not torqued or twisted)  and you continue to use good maternal positioning. Positioning without releasing tension in the abdominal soft tissues and pelvic joints isn't likely to succeed in rotating a baby in pregnancy, especially for a woman with a history of posterior babies or breech, because these positions show the uterine ligaments weren't in balance before pregnancy.

   
There are some myths about laboring with a posterior baby.
Let's start with appreciation for Penny Simkin's brave confrontation of a dogmatic trend that has arisen over the last few years.  At first, I was rather surprised to hear a talk by Penny Simkin called, "The OP Fetus; How little we know." Now, the light has gone on. Parents and professionals alike have some misunderstandings about the influence of posterior presentation on labor. For instance, "She didn't have back labor so I didn't think the baby was posterior." Or, "We did everything we could because we tried hands and knees position in labor."

Sometimes a midwife or doctor will say they don't pay much attention to a head down baby's position in late pregnancy because some posterior babies come out fine. Emphasis mine. Spinning Babies is about the 15-30% that need more help than strong labor and the hands and knees position.

 

“Transverse Lie” means a sideways position. The baby has his head to one of his mother’s sides and the bottom across her abdomen at her other side.

The word transverse is also used in phrases describing two of the normal head down positions. Left occiput transverse (the ideal starting position) and right occiput transverse. These head down babies facing the mother's hip. The side of the mother’s body that the back of the baby’s head is on is indicated by the first word, left or right. The baby faces the opposite hip. To see several different fetal positions go to Belly Mapping.