As with the maternal pelvis, the fetal head is made up of various mobile bones that form a variety of shapes and sizes. Because babies’ heads move in different directions, both on their bodies, and in relation to the mother’s pelvis, I’m going to break down the different ways we describe the fetal head.
“Attitude”
The first part of the baby’s head position to consider deals with “attitude”. This describes the amount of flexion or extension of the baby’s head, or how tucked the baby’s chin is. Why is flexion important? The amount of flexion going on will alter the diameter of the baby’s head as it moves through the pelvis, the cervix, and through the vagina. A cervix opens only as far as it needs to fit around the baby’s head. If the diameter is small, it will only need to dilate that amount. If the diameter is larger, then more dilation will need to occur. . A well-flexed head is a baby whose chin is tucked well into its chest, with the occipital (or back of the head) bone as the presenting part. From here there are varying degrees of flexion and extension.
Complete Flexion: this is the optimal position for a baby’s head to be in. The angle of the head and neck are less than 90 degrees, baby’s chin is tucked well into it’s chest, and the very back of the head presents first. The diameter of a completely flexed head, the suboccipitobregmatic diameter, is measured from just below where the occiput starts out to the center of the front fontanel. This diameter is about 9.5 cm
Moderate Flexion, or Military Attitude: with the military attitude, the baby’s head and neck are at a 90 degree angle, as if the baby is looking straight forward. The diameter of a military attitude, the suboccipitofrontal diameter, is measured from just below where the occiput starts to about the center of the forehead. This diameter is about 10 cm
Extended or Brow Presentation: this attitude has the baby’s head tilted back a bit, the angle of the head to the neck greater than 90 degrees. The diameter of brow presentation, the occipitofrontal diameter, is measured from the occiput start at the back of the head to about the eyes or the brow. The diameter is about 11.5 cm.
Hyperextended, or Face/Chin Presentation: this attitude has the baby’s head tilted way back and can be referred to as “star gazing”. The diameter of a hyperextended attitude, the mentovertical diameter, is measured from the tip of the chin to the highest point on the back of the head on the vertex. The diameter is about 13.5 cm.
As you can see, the variation in diameter of the fetal head is great in relation to how well flexed is the baby’s head.
Fetal Head Bones/Molding
So, can an unflexed head still fit through the pelvis and birth canal? Though it often takes more time and effort, yes, an unflexed head can still fit through!
Many pregnant women cringe when they think about pushing out a newborn baby’s head through their vaginas. How could something so large come out of something usually so small, they may wonder. While the majority of that topic can be turned into a blog post all on it’s own, for the sake of this blog post on OFP, we will discuss how the baby’s head can change size in order to fit through the mother’s pelvis.
We’ve already talked about how the attitude of the baby’s head alters the diameter of the presenting part. But, what if the baby comes down in one of those suboptimal positions, or what if the baby really does have a large head? The wonderful thing is, we were designed to deal with this as well. Just as the maternal pelvis can expand to allow more room, the fetal head can contract to take up less room.
The fetal skull is formed from five main bones – two frontal bones, which make up the forehead area; two parietal bones, which make up the majority o fthe skull; and the occipital bones, which makes up the very back of the skull, protecting the spinal cord.
The adult human skull has a static shape – it is not going to change unless the bones are broken. But from fetus up until eight years of age, the sutures between these bones have not yet fused. These unfused bones and the fontanels, or small gaps, between the bones, create a skull with the ability to shift, bones overlapping bones, reducing the diameter of the skull. This ability can even allow a malpositioned baby to fit through his mother’s pelvis. I’ve seen some pretty amazing coneheads in my practice! The ones that are always so amazing to me are those that cone at an angle, or off to the side of the baby’s head. It amazes me that we are able to adapt so well; that vaginal birth really is very accommodating!
Fetal Head Position
Fetal head position is different than the attitude of the head. The attitude deals with moving the head up and down from the neck. Position deals with the direction in which the head (and the body that follows) is facing.
When discussing fetal head position, the occipital bone – the one at the very back of the head – is always what is used as the point of reference. We describe the fetal head position as which direction of the mother is the fetal occipital bone pointed toward.
The front portion of the mother is called the anterior. The sides are called transverse. And the back portion of the mother is called the posterior. So, if the back of the baby’s head is pointed toward the front of the mother, the baby would be in an occiput anterior position
The various fetal head positions are as follows:
ROA (Right Occiput Anterior)
ROT (Right Occiput Transverse)
ROP (Right Occipital Posterior)
OP (Occiput Posterior)
LOP (Left Occiput Posterior)
LOT (Left Occiput Transverse)
LOA (Left Occiput Anterior)
OA (Occiput Anterior)
