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  • Apr 25 2014

    Mother’s & Baby’s Anatomy as it Pertains to Foetal Position

    Mother’s Birth Related Anatomy

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

    the bony pelvis forms the hard frame for the passageThe 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. A hormone called ‘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 Sacroilliac (SI) 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.


    Pelvic types

    There are four general pelvic types. Nearly half of Caucasian women have a Gynecoid pelvis while nearly half of women of African descent are said to have an Anthropoid pelvis (and also found to have a bit higher rate of Occiput Posterior babies at birth). About 1/4 of all women have an android pelvis, with it’s triangular inlet and a bit smaller outlet than its own inlet. 

    The variety of shapes, combined with the variety of fetal head presentations, plus size variations, mean that labors vary greatly. Here we see pelvic inlet shape and the correlating shape of the pubic arch at the outlet.

    Left to right in the drawing above:


    The Gynecoid pelvis has a roundish brim which encourages fetal rotation when the soft tissues and, especially, the broad ligament aren’t tight and twangy. The pelvic arch in front would allow three fingers to cover the urethra during a “potty dance” – the type of grab yourself and try not to pee your pants dance of a child waiting to get to the bathroom. Buttocks are round. Hip size doesn’t indicate the roomy inside and a petite woman can birth a large baby. When the pelvic floor and other soft tissues aren’t overly tight, the birth tends to go well and a posterior baby can rotate at several various phases of labor. 


    The android pubic arch may hang quite low, giving a fundal height reading higher than the compact bump may seem to justify. Closely set small buttock “muscles” of the android make small roundish or triangular cheeks to her “bottom.”  The android pelvis definitely has a 2 finger arch, rather than the 3 finger of the gynecoid.

    Posterior arrest is slightly higher for women with an android pelvis. Good fetal positioning, good flexibility in the pelvic joints and balance in the soft tissues help the natural labor progress. The posterior baby will hope to rotate before engagement or may not be able to rotate until the head fully passes the pelvis rotating on the perineum. Some posterior babies, the larger ones or if a mom can’t get out of bed to do some rotation exercises, will need a cesarean, even with a skilled baby spinner present. Manually rotating the baby’s head may be an option if a skilled doctor or midwife is present. A little help for the shoulders may be needed as the result of those women with low slung pubic bones. They may catch a shoulder.

    Tall women with average size babies often birth without an issue. I recommend flexible and balanced muscles before labor begins. Some women will need to start working out chronic pelvic torsion early in pregnancy or even before. 


    Long pelvis front-to-back, perhaps a narrow arch, perhaps not. Buttocks muscles look longer up and down than the round buttocks of a gynecoid. The Anthropoid pelvic arch can vary 2 or 3 fingers at the pubic arch. Measure 1/3 of the way south the clitoris towards the sitz bones.
    Common for breeches that don’t flip. More posterior babies who are born vaginally may be arriving through the anthropoid pelvis. Vertical maternal positions aid these fetal positions and help birth be more protected and finish by the mother’s own efforts. 


    The arch 4 finger arch of the platypelloid is quite wide. A woman’s hips may seem slightly wider side-to-side than her weight would demand. In other words, a thin woman with a platypelloid pelvis has her front and back quite close at her waist but her hips are wide. Her sitz bones are quite wide. Baby needs to be in the LOT position to get INTO the pelvic brim for engagement. Long early labor with strong contractions is possible. Once baby is into the pelvis labor tends to move along. Pushing may not be very long. 

    The soft tissues

    More important than pelvic shape is the state of relative balance found in the mother’s tone and symmetry of a woman’s uterine ligaments and related muscles.

    Our ability to stand depends on the psoas muscle pair. The psoas (pronounced so-as) begins at T-12 vertebrae 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.

    Side view of pelvis, uterus and some ligaments

    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.







    Molding of the fetal head during birthBaby’s Birth Related Anatomy

    The foetal 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 foetal 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 (an asynclitic birth or asynclitism refers to the position of a baby in the uterus such that the head of the baby is presenting first and is tilted to the shoulder, causing the fetal head to no longer be in line with the birth canal)

    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.


    • Spinning Babies
    • 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


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