The words "Optimal Fetal Positioning" may sound like technical jargon to many, but to my ears they are the sound of hope and relief. While my beliefs about birth are mostly rooted around the ideas of keeping hands off of the pregnant mama, and letting the laboring mama and the baby inside do the work they were made to do, understanding the way our bodies were designed can lend an enormous amount of insight into what makes labor work, and what doesn't.
When dealing with optimal fetal positioning, we mainly focus on two parts of the participating bodies. The mother's pelvis, and the baby's head. Now, of course, these body parts do not function on their own, and are intricately connected to the entire rest of the body; but for now, we're going to break it down into these two parts so that we can have a better understanding for how this works.
Part 1: The Pelvis
Most scientists will concur that the human birth process has evolved into a series of compromises. As humans became bi-peds, our pelves narrowed. In order to walk upright, we gave up the ability to birth with the ease of a wide pelvis; a pelvis that would essentially allow us to release our babies without any pushing. Whether you are of a scientific mindset, agreeing with the degrees of evolution that have brought humans to this point in history, or more of a faith-based mindset, agreeing that a Creator had a perfect plan for how a baby would come out of it's mother, the fact remains that this process works - when done right.
The pelvis is a complex and dynamic set of bones. It cannot be defined by one shape or size. It is both wide and narrow, short and long. And it has the ability to alter its shape! What is so wonderful about all of this, is that babies heads and bodies are still able to maneuver their ways through the various turns and twists of the pelvis, and still find their way out!
There are four main types, or shapes, of pelvises among humans, but they all have the same basic structure of the inlet, the cavity and the outlet. The diameters of each of these parts of the pelvis differ significantly.
The inlet of the pelvis is about 11-12 cm from front to back, and 13-13.5 cm from side to side. When I get to discussing the baby's head, you'll see why this is significant. For right now, I'm just talking about the structure of the pelvis.
The pelvic cavity is the narrower part of the pelvis. It has a more equally rounded diameter, each direction being about 12 cm wide. The ischial spines, the boney prominences that make up the narrowest part of the pelvis, are typically 11 cm apart.
The pelvic outlet is narrower side to side than it is front to back. The transverse diameter is about 11 cm. The anteroposterior diameter on an immobilized pelvis is about 11.5 cm. The amazing thing about the pelvic outlet is that the joints that hold the lower pelvic bones together are quite flexible. If a mama is allowed to be upright, and baby is able to press it's way through, the sacrum and the coccyx are able to move freely and will open up as the baby's head comes through the pelvis, creating an additional 3.5 cm of space!
Different Pelvic Types
With about 50% occurance, the most common pelvic type among women is the gynecoid pelvis. This pelvis type is the ideal shape for childbirth, with a rounded pelvic brim, blunt ischial spines, round sciatic notch and a pubic arch of about 90 degrees.
The next common pelvic type, occuring in about 25% of women, is the anthropoid pelvis. The pelvic brim on this pelvis is wider front to back than it is side to side, the ischial spines are blunt, sciatic notch is wide and the pubic arch is greater than 90 degrees. This type of pelvis is common among women who are tall and have narrow shoulders. Because of the shape of this pelvis, it is common to have posterior positioned babies.
About 20% of women have the next type of pelvis, the android pelvis. This pelvis is similar in shape to the male pelvis and tends to be more common in women who are short and stockily build. The pelvis is heart shaped, making it more likely to have a posterior positioned baby. Because the ischial spines are prominent and close together, the baby's descent may be slow late in labor.
And the final type of pelvis, occurring in only about 5% of women, is the platypelloid pelvis. This type of pelvis is kidney shaped, being much wider side to side than front to back. The ischial spines are blunt, and the sciatic notch is wide. Because of the width of this pelvis type, the baby may need to tilt its head in an asynclitic position in order to engage into the pelvis.
No matter which type of pelvis you have, each of these pelvis types are able to give birth to a baby! Why? In the next section you'll learn how amazing the fetal head is in relation to fitting in tight places.