We've been talking about how fetal position can affect a woman's belly shape. Today, we start discussing how it can affect labor.
As a reminder of what we discussed last time, remember that an Anterior baby (Occiput Anterior, or OA) means the baby is head-down; the back of the baby's head is towards mom's belly and the baby is facing towards mom's back.
A Posterior baby (Occiput Posterior, or OP) means that the baby is also head down BUT the back of the its head is towards mom's back and baby is facing towards mom's front. Other names for the OP position include "stargazer" or "sunny side up" because the baby seems to be looking "up" at the sky when mom is lying down.
Why Fetal Position is Relevant
Question: Why be concerned about the position of the baby before birth?
Answer: Because the anterior position (facing mom's back) is generally an easier position for birth. A posterior baby (facing mom's belly) is generally a harder position for birth and the cause behind many cesareans these days.
But why is that? Well, there are a number of factors that are at work here.
Speed and Ease of Labor
Generally speaking, anterior babies are born more quickly and easily than posterior babies.
Labor with an anterior baby is usually less painful and progresses smoothly (as long as the baby is lined up well and does not get a hand or arm in the way!).
Labor with persistently posterior babies tends to be slower and more painful, and more prone to stall partway through labor. (More on that below.)
With an anterior baby, the diameter of the baby's head that presents through the pelvis is smaller because the angle is different. The chin is usually tucked down so the smallest possible diameter of the crown of the head presents at the cervix. This then applies nice even pressure on the cervix so it usually dilates reasonably fast and easily.
The angle of presentation of the baby's head is different with a posterior baby, so the diameter of the head that must fit through first is larger. Furthermore, an OP baby often does not tuck his chin to his chest as much (sometimes called a "military" position), and its de-flexed head makes an even bigger diameter to fit through.
Some sources state that the average OA baby's head diameter is about 9.5 cm, compared to 11.5 cm for the same baby if he is OP and de-flexed. Two centimeters doesn't sound like a lot, but it's around 20% of the total diameter of the baby's head, which is significant. And that 20% can make a lot of difference in how quickly and easily the baby fits through the pelvis. (See the pictures below.)
In addition, the "fit" of the baby's head in mom's pelvis is trickier with an OP baby and there's not as much room for error. If the baby is a little out of alignment in an anterior position, he can usually still fit through. If he is a little out of alignment in a posterior position, the same baby may not "fit" as well.
Because the OP baby's head enters in a larger diameter and is often deflexed, it has to mold more.
"Molding" is where the bones of the baby's head slide over each other, like the metal plates of a vegetable steamer that fold in. This helps to make the head smaller and helps it fit through more easily. It's one of nature's ways to ensure that babies of diverse sizes can fit through pelvi of diverse shapes and sizes.
Molding is natural and no big deal, but there's quite a bit more molding needed with a posterior baby and that takes a lot of time. Alas, hospitals are notoriously impatient with "slow" labors.
Slower Dilation and Labor Dystocia
Because the pressure on the cervix with an posterior baby tends to be more uneven, dilation also tends to be slower and more uneven.
Furthermore, because the presenting diameter of the head is larger and needs more time to mold, a posterior labor often stalls out or slooooows down for a while in the middle of labor. This slowing/stalling is called "labor dystocia" and usually happens at about 4-7 cm.
Even though she is still only about partway through labor, during labor dystocia the mother may be showing classic signs of "transition" (the last part of labor) like shaking, intense pain, a premature urge to push, exhaustion, or wanting to give up.
If the baby needs to do extensive molding, this labor dystocia stage may go on for quite some time and can be very discouraging and tiring. As a result, a lot of women request epidurals (or even cesareans) at this point---often out of sheer exhaustion.
Because the posterior labor tends to be slower and may stall for a while, medical interventions are often used to compensate. Hospitals often try to speed things up by artificially strengthening contractions with drugs (pitocin augmentation) or by breaking the mother's water.
It should be noted that sometimes this does work just fine and may be better than doing nothing.......but sometimes it permanently jams the baby into a bad position that he can't get out of. His head gets stuck in that position, the pitocin augmentation keeps ramming him into the pelvis at a bad angle, and the pitocin may start reducing the amount of oxygen getting to him. As a result, he may go into fetal distress.
Also, once the mother's waters are broken, the cushion of fluid is lost. Labor usually becomes much more painful for the mother, the lack of cushioning fluid makes it more difficult for baby to rotate OA, and the baby has less protection, making him more vulnerable to fetal distress and infection. Many cesareans are done at this point because the baby is not tolerating labor well.
Furthermore, because of the uneven pressure on the cervix, there is often a cervical "lip" left near the end of dilation. Care providers often manually push this out of the way, over the baby's head----another quite painful intervention common to posterior labors.
It's no wonder that many moms with posterior babies are ready to call it quits before the baby is even born.
Labor with an OP baby tends to be more painful and difficult. The back of the baby's head tends to hit against the mother's sacrum, making labor more painful and concentrated in the back and pelvis (and sometimes the hips, if the baby tries to rotate to anterior). Back labor and OP positioning are not always connected, but they are frequent companions.
Requests for pain relief are more common with posterior babies because of the combination of back labor, labor augmentation drugs, breaking the mother's bag of waters, and the sheer exhaustion of a long hard labor. Many, many moms with persistently posterior babies end up with epidurals, even those who strongly desired natural childbirth beforehand.
Sometimes the epidural will help ease the pain from an OP labor enough to relax the muscles and help the baby turn anterior. However, because an epidural partially paralyzes the muscles of the uterus that help maneuver the baby through the pelvis, sometimes the epidural makes it even more difficult for the baby to turn anterior. So while sometimes an epidural can help in an OP labor, sometimes it's just another step along the way to a cesarean.
Ironically, the pain relief from epidurals tends to be less effective with an OP baby. There is often a need for frequent re-dosing of the epidural, and pain relief can be spotty, with "windows" of sensation. No one is quite sure why this happens, but it probably results from a lack of uniform distribution of epidural meds because of the pressure from the baby's head against the mother's spinal column.
Inadequate pain relief during a long, hard labor is another reason why mothers and hospital staff may be more quick to move to a cesarean. No one likes to suffer (or to watch others suffer), and a cesarean may seem like the most compassionate thing to do at that point....the best way "out" of a tough situation.
However, the spotty pain relief associated with epidurals in OP labors may make them less than 100% effective for the surgery itself....so there are no easy answers here.
Long Pushing Stages
Eventually, given enough time (or enough pitocin augmentation), many mothers of OP babies will dilate fully and begin pushing------only to have pushing go nowhere.
A long, painful pushing stage that goes on for hours is a classic sign of a posterior baby. The baby simply doesn't "fit" well in that position, or hasn't molded enough yet to get through.
However, with enough time and molding (or some creative pushing positions), some OP babies will finally "fit" through, hit the resistance of the mother's pelvic floor, and rotate to anterior. They are usually born very quickly after turning to OA.
Some babies remain persistently OP and are born face-up, but often to an exhausted mother and caregiver. They may have avoided a cesarean, but often at a price, because mothers of vaginally-born persistent OP babies tend to have more instrumental deliveries (low forceps or vacuum extractor), more episiotomies, and more (and more severe) perineal tears.
The good news is that they don't have the surgical recovery of a cesarean, nor a scar on their uterus that puts future pregnancies at risk----but it's still not usually an easy birth.
A better choice would be to find a way to prevent the OP position or to turn the baby to OA while in labor so that it could be born more easily, without the collateral damage from either a surgical birth or a rough vaginal one.
Illustrations of the Difference in "Fit"
Below are pictures of anatomical models of a fetal head and a woman's pelvis. In one picture, the baby is anterior (facing mom's back); in the next picture, the baby is posterior (facing mom's front).
Please note, it's the same fetal head model and the same pelvis model, but look how much more room there is around the baby's head when the baby is anterior.
Anterior fetal head
in woman's pelvis
Posterior fetal head
in woman's pelvis
Look one more time for good measure. Notice how much more room there is for the baby's head in the anterior position? Same baby, same head, same mother's pelvis.....but a different "fit" because of the baby's position and the flexion (tucking) of its head.
[Note for the curious: The lines drawn on baby model's head are "suture lines," where the plates of the baby's head bones come together. The diamond shape is the fontanelle or "soft spot" near the front of baby's head. Feeling for the shape of these suture lines is one way midwives and doctors figure out the baby's position during labor.]
Not All Posterior Labors Are Equal
It has to be noted that not all posterior births are difficult. Some women have easy OP births. They tell the story of how their posterior baby "flew out" and wonder why other posterior moms can't birth their babies so easily.
Well, it's because not all posterior labors are created equal.
Some posterior babies are relatively small and have their chins tucked nicely down; these labors take a little while longer than anterior labors on average but tend to be born vaginally. It's the big baby whose head is de-flexed and who is persistently posterior through all of labor that tends to have the most difficult labor and birth.
Many posterior babies flip to anterior partway through labor, once their heads have molded enough to get into the pelvis. These babies may have a slow start to labor, but labor usually progresses quickly once the baby rotates to OA.
Whether you've had a baby before can make a difference. Mothers who have given birth before (multips) tend to have more spontaneous OP vaginal births (no cesareans, forceps or vacuum extractors) than first-time mothers (primips). One Irish study showed that only 29% of primips had a spontaneous vaginal birth with an OP baby, while 55% of the multips had one. (Mind, that still means that even the multips had a 45% rate of major interventions with OP babies, which is still quite high.) Other research has found similar results.
Some posterior births happen when the baby begins labor anterior but then flips posterior partway through labor. Some research shows this can be associated with epidurals; the partial paralysis of the muscles in the area may make it more difficult for the baby to turn properly during its journey through the pelvis. However, a flip at this late stage (when it's already partway into the pelvis) usually means that even an OP baby comes out fairly efficiently and a vaginal birth is common.
It's the persistently posterior baby----one who is posterior from the beginning of labor and remains so consistently throughout, and especially one who is big and whose head is de-flexed----these are the posterior labors that tend to be long, hard, and painful.
That's not to say that a persistent posterior cannot be born vaginally. Of course not! A persistently posterior position is not an impossible position for birth.....but it often needs more time and patience and support from caregivers.
Unfortunately, because of economics and "standard of care" procedures, time and patience for longer posterior labors can be in short supply in hospitals. Therefore, the cesarean rate associated with persistently posterior babies can be quite high.
Research on Persistent Posterior Positioning
Here is a 2003 study that compared the effect of persistent posterior (OP) and anterior (OA) positioning on labor outcomes. There are several other studies like this available as well.
You can see that on average, OP babies had longer labors, longer pushing phases, and more than five times the cesarean rate.
Labor longer than 12 hours
Length of pushing greater than 2 hours
(Ponkey et al. Persistent fetal occiput posterior position: obstetric outcomes. Obstet Gynecol 2003.)
In addition, some studies find that persistent posterior positions are harder on the babies, with more NICU stays or longer hospital stays, more signs of stress, and sometimes lower Apgar scores at birth.
So you can see, persistent posterior positioning DOES affect labor and birth. It tends to create longer, harder and more painful labors, long pushing phases, and results in a much higher rate of cesareans. It also can be more stressful on the baby.
Posterior positions that occur partway into labor, posterior babies that rotate to OA during labor, posterior babies in multips, or posterior babies that tend to be small and/or with well-tucked heads.......these OP babies don't usually cause as many problems and have a good chance at a normal vaginal birth.
But babies that stay persistently posterior from before labor and throughout labor....these are associated with a lot more difficulties in labor and birth, especially when the baby is big and/or its head is de-flexed. They have a much higher risk for birth via cesarean, forceps, or vacuum extractor.
So now the question arises......What, if anything, should we do to try and prevent persistent OP babies? Is there any effective way to prevent or turn persistent OP babies?
Or is persistent OP positioning simply another variation of normal in labor, and all that is needed is a little more time and patience?
(Ahhh, the $64,000 question!!)
Tune in next time.......that's another post in the series!!