Here is a brief description of anatomy as it pertains to fetal position.
We’ll go over the mother’s anatomy and then the baby’s.  To see how they work together during birth, see The Birth Process in the section on Birth.

To see a new window with birthing terms and initials, click birth glossary.

Mother’s Birth Related Anatomy

A woman’s birthing anatomy includes soft tissues and hard bones.

The bones

Fortunately, our bones are held together by flexible tendons. In pregnancy, these joints become more mobile.  Waddling is an example of what happens when these joints get softer. The hormone relaxin helps make the pelvis a dynamic, flexible passageway.

The bony pelvis has four joints. In the front of the pelvis is the symphysis pubis joint. Movement here really isn’t that comfortable. Sometimes a pregnancy belt holds this joint stable for walking and rolling over in bed. Symmetry in the symphysis pubis (pubic bone) reduces spasm in the round ligaments and helps the sacrum, around back, to be aligned properly.

The symphysis pubis

On either side of the sacrum are the SI joints (Sacroilliac joints). 

These are located  where the dimples are. Many plastic baby dolls have SI dimples above their bum.  The SI joints are a common location for aches when the pelvis is weak or crooked.
Symmetry in the SI joints will help the sacrum be lined up with the pelvic brim. Then the baby can get into a nice, head down position. A chiropractic adjustment helps get the symphysis and the SI joints aligned.

The sacrum, rather than fused, is slightly mobile and in the birth process actually moves to allow the head past.
The tailbone is connected by a joint to the lower end of the sacrum. Sometimes this needs an adjustment, too, especially after birthing a baby. Ligaments connecting to the sacrum and tailbone (coccyx) will become more symmetrical and their tone will be more relaxed and less in spasm after bodywork on the pelvis.

The soft tissues

Our ability to stand depends on the psoas muscle pair. The psoas (pronounced so-as) begins at T-12 vertebre and sweeps around from the center of the sides of the spine over the pelvis to attach at the top of the thigh bone (femur). The muscle pair comes around like supporting arms, pulling up the legs so that our backs don't fall over.

As the psoas comes across the pelvis, its makes a diagonal support for our organs. The support can be thought of as a shelf. When the uterus in large, at the end of pregnancy, a tight psoas can hold up a baby from descending and engaging in the womb. 

Many discomforts of the abdomen can stem from psoas tightness. There are exercises to release the psoas. A great source is Liz Koch's The Psoas Book, available at coreawareness.com

So, as the psoas is balanced, so goes the birth.

The psoas is the lower triangle (pointing up) of two great muscle pair triangles that give core strength to the human body. The upper triangle (pointing down) is the trapezius, which is more of a diamond shape really, but I say two opposing triangles to help you to visualize of the polarity or pull between them to support our bodies. (Illustrations to come.)

The psoas additionally effects our pelvis and uterus because it shares the tendon connecting it to the thigh with another muscle pair, the iliacus. Together they team up to form the iliopsoas muscle group. The tone of the iliacus is dependent in part by the tone of the psoas. So, as the psoas goes, so goes the iliacus. This muscle spans from the top of the thigh (lesser trochanter) back over the pelvic brim to attach at the inside edge of the ilium (behind the hip bone but not as far to the center as the sacrum).

The womb is supported by a series of ropes and slings called ligaments and fascia.

The ligaments of the womb have a unique mixture of fibrous tissue and muscle cells. The muscle cells allow the ligaments to become longer during pregnancy so that the ligaments can grow with the uterus.

Symmetry of the ligaments helps the womb be held upright. The cervix will be aligned properly, first

aiming back in pregnancy and then, during birth, lined up with the birth canal. Dilation is less painful when the cervix is not held to the side or back by spasming cervical ligaments. The baby’s head is better positioned with symmetrical ligaments because the lower uterine segment is not in a twist.

Some spiraling and leaning to the right is considered normal for the uterus. But too much lean is not helpful for optimal uterine functioning, including birth.

The fascia is a membranous tissue that wraps every muscle, organ and bone in the body.  The fascia moves with the moving body but also seems to store the “memory” of an abrupt halt. Whether that sudden stop has to do with a trauma or a long-time habit of poor posture, the fascia can get a wrinkle in it that pulls organs and bones out of alignment or symmetry.

Fortunately, craniosacral therapists, myofascial therapists and, to a lesser degree, chiropractic adjustments, can help to release the fascia and so, bring about a greater symmetry of the body.

Baby’s Birth Related Anatomy

The fetal head is heavy in comparison to the rest of the baby’s body. The vertical positions of walking, standing and sitting help the heavy head settle lower than the body during the third trimester, and sometimes in the second trimester.

The fetal skull has not yet hardened and remains somewhat flexible, and moldable, for fitting through the pelvis. There are plates of bone and cartilage that are nearly finished coming together at birth. That nearly finished margin is what allows molding.  These margins are called sutures.
The skull plates are held together by a coating for strong fascia. This membrane also wraps down the spine to the pelvis and legs. The fascia also connects to membranes that support the brain, called the tentorium cerebelli.

Which angle the head presses past, or onto, the bony pelvic passageway determines molding. When the crown of the head enters the pelvis first molding is most efficient.  When a plate, rather than the margin between, or sutures, aims into the narrow part of pelvis, molding takes a long time and does less to reduce the diameter of the baby’s head. One example of this is the asynclitic baby. Second stage can take a long time and pushing can be quite strenuous when a baby is asynclitic.

The baby’s shoulders can also mold a bit for the birth process. The shoulder girdle is flexible and many times the shoulders are folded towards the chest for the actual emergence. Other times one shoulder comes out just ahead of the other in another natural variation to reduce shoulder breadth.

References:

Holistic Midwifery, Vol. II, Anne Frye
Hands of Love, Carol Phillips
The Female Pelvis by Blandine Calais-Germain
A New Look at a Woman’s Body by Boston Womens’ Health Collective
Gray’s Anatomy by Henry Gray