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.

  

The anterior baby's head enters the pelvis from the crown of the head first. The crown molds more easily.  The anterior baby's head measures smaller than the posterior baby's head when coming through the pelvis.

There are four posterior positions. The direct OP is the classic posterior position with the baby facing straight forward. Right Occiput Transverse (ROT) is a common starting position in which the baby has a bit more likelihood to rotate to the posterior during labor than to the anterior. Right Occiput Posterior usually involves a straight back with a lifted chin (in the first time mother). Left Occiput Posterior places the baby's back opposite the maternal liver and may let the baby flex (curl) his or her back and therefore tuck the chin for a better birth. These are generalities, of course.

See more about posterior positions in Belly Mapping or in the other articles in this section.

There are a spectrum of effects possible with a posterior baby. The same effects do not happen to all women. First let's look at the effects of a posterior fetal position and then we'll try and figure out who is likely to have which effects.

The possible effects range from 

• Longer pregnancy

• The amniotic sac breaking (water breaks, membranes open, rupture of membranes) before labor

• Start and stop labor pattern

• Longer early labor
• Longer active labor
• Longer pushing stage
• (Maybe a woman has all three phases of labor lengthened by the OP labor, or one or two of the three phases listed.)
• More use of vacuum or forceps
• More likely to tear
• Sometimes the baby's head gets stuck turned half way to anterior - in the transverse diameter. This can be called a transverse arrest. It is not a transverse lie
• More likely to need a cesarean

These effects are in comparison to a baby in the left occiput anterior or left occiput transverse fetal position at the start of labor.

Who is likely to have a hard time with a posterior baby?

• A first time mom, or
• A first time mom whose baby hasn't dropped into the pelvis by 38 weeks gestation (two weeks before the due date).
• A woman with an android pelvis ("runs like a boy," often long and lanky, low pubis with narrow pubic arch and/or her sitz bones are close together, closer than the width of a fist).
• A woman whose baby, in the third trimester, doesn't seem to change position at all, over the weeks. He or she kicks in the womb and stretches, but whose trunk is stationary for weeks. This mother's broad ligament may be so tight that she may be uncomfortable when baby moves.
  
• A woman  who has an epidural early in labor, before the baby has a chance to rotate and come down.
  

• A woman who labors in bed

• A woman who lacks support by a calm and assured woman who is a bit older than her or is otherwise calming and reassuring to the birthing mother.
  

Who is likely to have an easy time with a posterior baby?

• A second time mom who's given birth readily before (pushing went well)
  
• Someone who's posterior baby changes from right to left after doing inversions and other balancing work, though the baby is still posterior
• A woman with a baby in the Left Occiput Posterior
• A woman who gets body work, myofascial release, etc.
• A woman' whose baby engages
• And of all of these, what is necessary is a pelvis big enough to accommodate the baby's extra head size
• A woman who uses active birthing techniques; vertical positions, moves spontaneously and instinctively or with specific techniques from Spinning Babies and other good advice.

Other women may also have an easier time than public opinoion might indicate, too, just because she isn't on this list, orjust because she is on the "hard" list, doesn't mean she will have a hard time for sure. These are general observations, but are not either condemnations nor promises.

What causes a  baby to be posterior?

There is a rising incidence of posterior babies at the time of birth. We know now that epidural anesthesia increases the rate of posterior position at the time of birth from about 4% for women who don't choose an epidural in a university birth setting up to about 13% when an epidural is used (Lieberman, 2005)

Most babies who are posterior early in labor will rotate to anterior once labor gets going. Some babies rotate late in labor, even just before emerging. Studies, such as Lieberman's, show that at any given phase of labor, another 20% posterior babies will rotate so that only a small number are still posterior as the head emerges.

My observations are that the majority of babies are posterior before labor. The high numbers of posterior babies at the end of pregnancy and the early phase of labor is a change from what was seen in studies over ten years old. Perhaps this is from our cultural habits of sitting at desks, sitting in bucket seats (cars), and leaning back on the couch (slouching).

Soft tissues, such as the psoas muscle pair or the broad ligament,  also seem  to be tight more often from such posture, from athletics (quick stops, jolts and falls), from accidents and emotional or sexual assault.

Being a nurse or body worker who turns to care for people in a bed or on a table will also twist the lower uterine segment (along with some of the previously mentioned causes). This make s the baby have to compensate in a womb that is no longer symmetrical.

Less often, the growing baby settles face forward over a smaller pelvis, or a triangular shaped pelvis (android). At the end of pregnancy the baby's forehead has settled onto a narrower than usual pubic bone, if tight round ligaments hold the forehead there, the baby may have a tough time rotating. These are the moms and babies that I'm most concerned with in my work at Spinning Babies.

A baby that was breech beyond week 30 -34 of pregnancy will flip head down in the posterior position.

A woman with a history of breech or posterior babies is more likely to have a breech or posterior baby in the next pregnancy. However, she may not have as long a labor, even so.

She's a busy little thing and still breech. I've delivered a posterior baby and don't want the birth of a breech baby on my resume too! So I'm praying that she'll get into the head down-face down position! - TexasNeals

I'll be adding references to this article in the future. you can email me for a reference sheet and bibliography. Read the 3 Principles next.