Woman, know thy pelvis

Pelvic explorations begin with a tour of the basic structure of the pelvis. For me, anatomy pictures aid in my ability to visualize what is going on inside of my body whether in a yoga pose or stuck in my car on a commute.  I hope they prove helpful to you as well.

The pelvis is the bony support of the perineum.  It is made up of four bones which are arranged like a bowl: the two innominate bones, the sacrum, and the coccyx.

The innominate bones are formed by the fusion of three bones – the illium, ischium, and pubis.  The illium is what we often think of as our hip bones and what people often are talking about when they say something like “she has childbearing hips” – a myth we’ll unpack later. The ischial tuberosities are the lower part of the ischium bones and are our “sitting bones”.  The pubis bone is the pubic bone, at the front of the pelvis. The two innominate bones are a mirror image of each other, one on each side of the pelvis. In the illustration below, the labels are all on the left side of the pelvis (your right side facing the image) which is one innominate bone.

The innominate bones both attach to the sacrum, which is a triangular bone at the back of your pelvis.  The coccyx is also known as the tailbone.  Both the sacrum and the coccyx have fused vertebrae – five in the sacrum and four rudimentary vertebrae in the tailbone.

The bones are united by four joints.  The two sacroiliac joints connect the innominate bones to the sacrum.  These joints are easily stressed during pregnancy and cause many women low back pains and aches. The sacrococcygeal joint (not shown in the drawing) is a synovial hinge joint between the fifth sacral and first coccygeal vertebrae which allows both flexion and extension. Extension increases the diameter of the outlet of the pelvis and thus plays an important role in birth. Did you know that if a woman is allowed to be upright during pushing, the sacrum and the coccyx are able to move freely and will open up as the baby’s head comes through the pelvis?  Overextension during delivery may break the small cornua by which the coccyx is attached to the sacrum. It is vitally important that a woman birth in a position that does not impede the movement of the sacrum and coccyx.  More on this in a future post. Lastly, there is the pubic symphysis, a cartilaginous joint which connects the two sides of the pubic bones.

These four joints, influenced by the hormones progesterone and relaxin, are what allow the pelvis to widen during pregnancy and birth. There is also increased flexibility of the sacroilliac joints and the pubic symphysis.  Hyperemia and softening of the ligaments around the joints takes place also.  Excessive mobility of the pubic symphysis can lead to extreme pain and difficulty in walking. Belly binding, either with a product made for use during pregnancy or with a large scarf, can be very helpful for women experiencing pelvic pain. Regular bodywork is helpful, especially in the last trimester  – chiropractic or cranial sacral are both good options, as is massage. It is important to be aware of this extra flexibility during yoga as it is easy to overstretch and end up with low back pain.

True Pelvis | False Pelvis

So back to the “childbearing hips” myth.  You know, the one where if you have wide hips it means you can easily birth a baby? What we think of as our hipbones are the illium part of our innominate bones.  The spacing of these bones has nothing at all to do with birth, and is in fact, the “false pelvis”.  The “true pelvis” is the what the baby descends into and navigates through.

The drawing below shows us the false pelvis on the left, and as you can see, the width of the hips has nothing at all to do with the actual bowl of the pelvis.  The true pelvis lies below the pelvic brim, and is shown in the drawing on the right.  It is composed of three parts – the pelvic brim, also known as the inlet; the pelvic cavity; and the outlet (best view is the third drawing).

What you may notice is that each part of the true pelvis has different diameters and shapes.  The inlet is wider from side to side than it is front to back; the cavity is a curved canal, and the outlet is diamond shaped.  The reason I mention this is there is a relationship between the presenting part of the baby (usually the head) and the pelvis that is important to be aware of.

 As you can see by the drawing above, the baby’s head would fit nicely through the outlet in this position.  However, it would have a hard time descending into the inlet, never mind navigating the curved bowl of the pelvic cavity.  The baby must enter it’s head into the pelvis in a transverse position, navigate the curved cavity, and rotate to the vertical position to birth out of the outlet.  All of this will be expanded upon in a different blog post, but the main point I want to get across is the baby is an active participant in the birthing process. It is important to remember that  the pelvis is not rigid during pregnancy, labor & birth because of the joints and hormonal influence on the joints.

Mapping your own pelvis

Certain parts of your pelvis can be located externally, allowing you to form a mental image of your own pelvis.  Try it now!

On each side, in front, you can feel the illium bones.  The top of these bones, the iliac crest, is where you put your hands on your hips.  It is curved.  Put your hands on the curve and follow it down toward the abdomen. Feel that bony prominence where the iliac crest ends? This is called the anterior-superior iliac spine (ASIS).

In front, beneath the hairline, is the pubic region where the two pubic bones are joined together by the thick fibrous cartilage of the pubic symphysis.

At your low back, you can touch your sacrum, right there in the middle. You can also feel the coccyx at the extreme end, below the sacrum.  If you press through your glutes to either the left or right you can feel the bony ichial tuberosities, aka sitting bones.

Sitting on a firm chair, put both hands on the ilac crests.  Notice the contact made between your ischial tuberosities and the chair. This gives you an idea of the height of your own pelvis.  Form a mental image of its funnel-like shape from top to bottom.

Now, put one hand on your sacrum and the other on the pubis.  You can get a sense for the distance between the front and the back of the pelvis.

These reference points will give you an idea of the exterior dimensions of the pelvis, and will also begin help you visualize the interior dimensions.

(thanks to Blandine Calais-Germain and the fantastic book The Female Pelvis, Anatomy & Exercises for the above exploration exercise!)

Next post – the perineum and soft tissues (muscles & ligaments) of the pelvis!

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