Mother’s Birth-Related Anatomy

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

pregnant swimmer under water

The bones

Our bones are held together by flexible tendons. In pregnancy, these joints become even more mobile. Waddling is an example of what happens when these joints get softer. The baby passes through the mobile pelvis.

The hormone relaxin helps make the pelvis a dynamic, flexible passageway.

4 joints of the pelvis

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.

On either side of the sacrum are the SI joints (Sacroiliac 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.

Four pelvic types

Pelvic types

Four general pelvic types are taught in midwifery and obstetrical schools. Caldwell-Moloy (1933) taught that nearly half of Caucasian women have a Gynecoid pelvis (rounder at the inlet, but wider side-to-side and a little less room front-to-back) while nearly half of women of African descent are said to have an Anthropoid pelvis (oval at the inlet, roomiest front-to-back of all pelvic types). Today there is new evidence that most people share a mix of types. The conclusions of researchers can have bias of their era and cultural training.

For instance, today an android pelvis is described as a more narrow outlet and more narrow “front of the pelvis” and narrower pubic arch. The word android is going out of style. See the pubic arch in the drawing above that has room for only two finger widths across and a triangular arch shape (the second option)?

While the terms are being sorted out, I admit I haven’t updated the four types. In practice, I find the bone shapes are relevant even if I don’t share the assumptions of the researchers.

I do note that birth progress can reflect the pelvic shape. Understanding inlet and outlet diameters can help match appropriate techniques and strategies for labor progress.

Only about 5% of women are said to have a platypelloid pelvis. The fetal position of LOT is crucial for engagement.

Kuliukas (2015). looked at 64 women and did not find a clustering of four types but a range throughout.

Some research is suggestive of pelvic shape, especially width, having to do with physical activity habits in childhood.

Pelvis width associated with bone mass distribution at the proximal femur in children 10–11 years old.

The variety of pelvic shapes, combined with the variety of fetal head presentations, plus size variations, mean that labors vary greatly. Engagement is the phase of labor most affected by pelvic type.

In the drawing above, we see pelvic inlet shape and the correlating shape of the pubic arch at the outlet.


The gynecoid pelvis has a roundish brim which allows fetal rotation when the muscles and ligaments in and around the pelvis aren’t tight and twangy.

gynecoid pelvis xray radiopaedia

The pelvic arch in front would allow three fingers to cover the urethra during a “potty dance” – the type of grabbing 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.


Narrow in the front, this pelvis occasionally catches the posterior baby's forehead. Back it up and let it turn!

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.

Narrow in the front, this pelvis occasionally catches the posterior baby’s forehead. Back it up and let it turn!

The 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.

Anatomy drawings by Gail Gynecoid or Android

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

Recently a woman told me her doctor felt she may not be able to have a vaginal birth after she previously had a cesarean for birthing her first child. She asked me what I thought. While I acknowledge there are more challenges with an android pelvis for some of these labors, most births through an android pelvis are going to be able to finish by the woman’s own efforts -and her baby’s. I said:

I do know that women in your shoes…in your hips… do give birth every day. 24% of Caucasian women have android pelvis, and, almost that for women of African descent. And these womens’ great great grandmothers were birthing their great grandmothers, and their grandmothers were birthing their mothers, and one of them birthed you.

Adding body balance and helping baby into position is extremely important when birthing with a smaller and usually triangular or “android” pelvis shape. If the presenting angle of the fetal head makes the head seem too large for her body, a quickly progressing labor will bring a surprise ending. After what seems like a normal first part of labor, there can be a long time with no further fetal descent. Cesarean surgery can be a tool for finishing this labor situation but shouldn’t be a first choice when body balancing and birth positioning could make a positive solution for many in this situation. If the posterior baby can back up to turn to left occiput anterior and try again, there is much reason to expect a vaginal birth. To help the baby do this, Side-lying Release followed by an inversion of some type during a contraction or two can be necessary.


Anthropoid pelvis

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.

The arch could be a narrow 2 fingers or a wider 3 fingers at the pubic arch (as shown here in the illustration). Measure the pubic arch about 1/3 of the way between the clitoris and the sitz bones, rather than the very top.

Common for breeches that don’t flip. More posterior babies who are born vaginally may have the advantage of the anthropoid pelvis’ longer front to back opening. Various vertical maternal positions and movements protect fetal engagement. Understanding the unique benefits of this pelvic type help birth finish by the mother’s own efforts.


The pubic arch is a wide 4 finger span in the platypelloid, which 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 wide hips but her pelvis from front and back is quite narrow. The sitz bones are quite wide apart, more than the width of the person’s fist (if she can reach between them while lying down).

Baby really needs to be in the LOT position to get INTO the pelvic brim for engagement. Long early labor is common, but if the baby isn’t LOT, the two days of labor will be all about getting baby rotated and strong contractions and mobility are essential. Once the baby is into the pelvis labor tends to move along, within 5-8 hours of engagement. Pushing may not be very long because the outlet of the pelvis is large. Using Daily Essentials: Activities for Pregnancy Comfort & Easier Childbirth, and Spinning Babies® Parent Class may help your baby get into a LOT or other ideal position.

four pelvic cavitiesThe pelvic cavities of Gynecoid, Anthropoid, Android (narrowest at the bottom) and Platypelloid (widest at the bottom, narrow at the front to back at the top)

How to fit the pelvic shape into a label or category is less important to me than having a set of skills to identify the relationship between the long axis of baby’s head with the short distance of the pelvic level where baby waits during a labor stall.

Knowing what is going on between the baby and the mother at the inlet or other diameters of the pelvis and what we can do to help when help is indeed appropriate. This is explained in the low-cost download, Spinning Babies® Quick Reference.

Pelvic Shape and lack of engagement

Occasionally the pelvic shape sets the fetal position. But this, I believe, is secondary to soft tissue “balance.” The android pelvis and platypelloid pelvis don’t accommodate posterior presentations. The baby must rotate or will not drop into the pelvis. But the baby in around pelvis shape (gynecoid) may. A baby in the Anthropoid pelvis (longer front to back) can fit while Occiput Posterior (given average muscle balance) When the mother can re-angle her pelvis to let the baby in (See steep inlet in Is baby engaged?). A birth stool, posterior pelvic tilt, and standing and leaning over a dresser feels right.

In the Android pelvis, the baby must be LOA for optimal engagement. In the platypelloid, the baby will engage from Left Occiput Transverse before they can engage in pregnancy or in labor because of this pelvic shape. In these two pelvises, there really is an optimal fetal position for engagement. Pelvic shape categories are like points of direction, they are useful for orientation to the practitioner’s choice of solutions (as opposed to recent recommendations to skip labor and go to surgical birth). But by far, most people don’t live due North, South, East or West. And most of us don’t have a “pure” pelvic type but a mixture depending on fascial forces during gestational development and learning to walk.

Rarely, a pelvis is too small to let the baby enter the pelvis. When a pelvis is too small its called CephaloPelvic Disproportion (CPD, or baby’s too big). Rickets, injury or an actually too-big-baby may be the cause here. CPD is rare but does exist.

Before we suspect CPD in pregnancy, check to see if the baby is engaged. Until the baby is LOA or LOT and in one of those ideal starting positions, it’s too soon to call it CPD.

The soft tissues

Psoas w model

So as I was saying… The Psoas.

More important than pelvic shape is the tone and relative symmetry of a woman’s uterine ligaments and muscles.

Our ability to stand depends on the psoas muscle pair. The psoas 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, it makes a diagonal support for our organs. The support can be thought of as a shelf. When the uterus is 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, but there are exercises to release the psoas. A great source is Liz Koch’s The Psoas Book.

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.

The psoas additionally affects 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).

Pelvic floor

Pelvic Floor drawing

The pelvic floor is three layers of several muscles that rise into the lower pelvis to uplift the uterus, bladder, and other abdominal organs. The pelvic floor is open front to back to allow the urinary urethra, vagina, and rectum through to the body’s exterior. The lovely thing about the pelvic floor is the benefits of balance has far reaching benefits from better fetal positioning, easier and less painful birthing, better control and ease in peeing and pooping, walking, thinking, mood, breathing, and sexuality. Some benefits are very direct and others are secondary. A tight pelvic floor is not typically the best functioning, so beware of over kegeling.

The pelvic floor should be considered as one unit since the three compartments work as one entity.

Uterine shape

The uterus is pear shaped before the first birth and apple shaped for future births, Jean Sutton, New Zealand midwife describes. The uterus is shaped to support a baby in a vertical position, and head down is common due to gravity.

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. These are called dextrorotation and right obliquity.

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, moving forward to line 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.


The fascia is a 3D web of fibriles which are in constant movement. This web forms 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 thicken and then pull which can, over time, pull organs and bones out of alignment or symmetry. The function of the fascia in balance can be the most important contribution to overall health.

Fortunately, craniosacral therapists, fascial therapists and, to a lesser degree, chiropractic adjustments, can help to release the fascia and so, bring about a greater symmetry of the body. See more at Adrienne Caldwell’s blog. She shares insights and references about the fascia in her post which also talks about scar tissues after cesarean section and how multi-directional massage may reduce obstruction in motion and health.

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 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.


  • Holistic Midwifery, Vol. II by Anne Frye
  • Hands of Love by 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
  • Kuliukas, A., Kuliukas, L., Franklin, D., & Flavel, A. (2015). Female pelvic shape: Distinct types or nebulous cloud?. British Journal of Midwifery, 23(7).
  • Cardadeiro, G., Baptista, F., Janz, K. F., Rodrigues, L. A., & Sardinha, L. B. (2014).
  • Journal of bone and mineral metabolism, 32(2), 174-183.
  • Maccioni, F., & Alt, C. D. (2018). MRI of the Pelvic Floor and MR Defecography. In Diseases of the Abdomen and Pelvis 2018-2021 (pp. 13-20). Springer, Cham.

You may also like to look at Amy Hoyt’s blog posts on Optimal Fetal Positioning with explanations and photos of the pelvis and baby (doll) on her blog: Natural Birth in Kitsap.

Daily Essentials
Daily Essentials can be practiced daily throughout pregnancy to help bring balance and comfort — and an easier, shorter birth.
Spinning Babies® Parent Class
Spinning Babies® Parent Class provides clear instructions on how to use Spinning Babies® for a more comfortable and confident pregnancy and labor.

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