The pelvis opens during childbirth through a coordinated process that involves hormonal softening of ligaments, movement at three key joints, repositioning of the sacrum and coccyx, and dramatic stretching of the pelvic floor muscles. None of this is a single event. Months before labor begins, hormones are already loosening the connective tissue that holds the pelvic bones together, and during labor itself, the pelvis reshapes in real time as the baby descends, with different dimensions widening at different stages. Maternal position plays a surprisingly large role in how much room the pelvis can create.
The Joints That Make It Possible
The pelvis is not a single fused ring. It is made up of three bones held together at three joints: two sacroiliac joints at the back, where the sacrum meets each ilium, and the pubic symphysis at the front, where the two pubic bones connect through a disc of cartilage. In everyday life, these joints barely move. The sacroiliac joint has a range of motion of only about 3 degrees in flexion and extension, with even less in rotation and side-bending, because the sacrum is wedged tightly between the hip bones and reinforced by strong ligaments.1PubMed Central. Biomechanics of the Sacroiliac Joint: Anatomy, Function, Biomechanics, Sexual Dimorphism, and Causes of Pain The pubic symphysis, similarly, allows very little movement in a non-pregnant person.
Pregnancy changes the rules. The female sacrum is already wider, less curved, and more tilted backward than the male sacrum, and women’s sacroiliac joints exhibit greater mobility and higher ligament strains even outside of pregnancy.1PubMed Central. Biomechanics of the Sacroiliac Joint: Anatomy, Function, Biomechanics, Sexual Dimorphism, and Causes of Pain During pregnancy, the small amount of movement that these joints allow becomes the margin that makes vaginal birth physically possible. The widening at the pubic symphysis and the rocking of the sacrum at the sacroiliac joints are the primary bony mechanisms through which the pelvis gains extra space.
How Hormones Prepare the Pelvis
The loosening process starts well before labor. The hormone relaxin, produced mainly by the ovaries and placenta, induces loosening of pelvic ligaments and joints during pregnancy.2PubMed. Circulating levels of relaxin are normal in pregnant women with pelvic pain Relaxin works by activating collagenase, an enzyme that breaks down collagen fibers in cartilage and tendons, making those tissues more pliable. It also plays a role in bone remodeling and ligament healing.3PubMed Central. The effect of relaxin on the musculoskeletal system Estrogen and progesterone contribute as well, increasing the water content of connective tissue and further softening the cartilage at the pubic symphysis.
This hormonal softening is what allows the pubic symphysis to widen during delivery and permits the sacroiliac joints to accommodate greater movement than they normally would. At the time of childbirth, natural widening of both the pubic symphysis and the sacroiliac joints occurs, and these changes are usually reversible afterward.4PubMed Central. Peripartum Pubic Symphysis Diastasis-Practical Guidelines The result is a pelvis whose joints can give a few extra millimeters to centimeters in critical dimensions, which matters enormously when the space is already tight.
Nutation and Counternutation of the Sacrum
One of the least intuitive aspects of pelvic opening is that the pelvis does not simply expand uniformly. The sacrum rocks back and forth during labor in two opposing movements, and each one opens a different part of the birth canal at a different time.
When the baby is entering the pelvis from above, the sacrum tips backward at the top relative to the hip bones. This movement, called counternutation, widens the pelvic inlet, the upper opening through which the baby’s head first drops into the pelvis.5PubMed. Movements of the pelvic bones of expectant mothers during vaginal delivery Later, as the baby descends through the mid-pelvis and approaches the outlet, the sacrum tips forward at the top. This is nutation, and it widens the lower part of the pelvis, including the plane of the pelvic outlet, making room for the baby to emerge.5PubMed. Movements of the pelvic bones of expectant mothers during vaginal delivery
A finite element simulation explored how the rotation of the femurs (thigh bones) affects this sacral movement. After about 6 degrees of external femur rotation, the pelvic inlet area increased by roughly 11 square centimeters, from 130 to 141 square centimeters. The study found that the sacroiliac ligaments, rather than the bone-on-bone fit, are what drive counternutation, and that combining external rotation with flexion of the thighs could be even more effective at opening the inlet.6PubMed. The impact of femoral rotation on sacroiliac articulation during pregnancy. Is there evidence to support Farabeuf’s hypothesis by finite element modelization? This finding has practical implications for birth positioning, which we will get to shortly.
The Coccyx and Pelvic Floor Give Way Too
The bony opening is only part of the story. The coccyx, or tailbone, deflects backward as the baby’s head pushes against it during descent. MRI imaging performed during actual labor has captured this happening: posterior coccygeal deflection occurred in every woman studied, with the distance between the coccyx and the pubic bone increasing by about 0.8 centimeters on average.7PubMed Central. MRI visualization of maternal pelvic and perineal anatomical changes during the second stage of labor: a feasibility study That same MRI study showed the iliococcygeal portion of the levator ani muscle flattening and actually reversing its curvature under the pressure of the fetal head, corresponding to the visible perineal bulge. The bladder was displaced upward as the baby descended.
The soft tissue stretching involved is extreme by any biological standard. A simulation study found that the medial portion of the pubococcygeus muscle, the shortest and most central part of the levator ani, stretches to about 3.26 times its resting length during delivery. Other parts of the levator ani reach stretch ratios of roughly 2.3 to 2.7 times their original length, and the degree of stretch scales directly with fetal head size: a 9 percent increase in head diameter produced a 9 percent increase in muscle stretch.8PubMed Central. Levator ani muscle stretch induced by simulated vaginal birth In vivo MRI of a live childbirth confirmed substantial circumferential stretching, with the lateral diameter of the levator hiatus increasing by about 32 percent and the transverse diameter by about 39 percent.9American Journal of Obstetrics and Gynecology. In vivo evidence of significant levator ani muscle stretch on MR images of a live childbirth
Computational models predict that even an uncomplicated vaginal delivery inflicts some degree of injury to the pelvic floor muscles, with more than 10 percent of muscle fibers sustaining damage, particularly during the extension phase of the fetal head.10PubMed. Numerical simulation of the damage evolution in the pelvic floor muscles during childbirth This is not a sign that something went wrong. It is simply the mechanical reality of fitting a baby’s head through a tight space.
The Baby’s Head Adapts Too
Pelvic opening is a two-way negotiation. While the pelvis expands and the soft tissues stretch, the fetal skull also reshapes. The unfused bones of the baby’s skull overlap and compress during passage through the birth canal, a process called molding. A numerical simulation of this process found that fetal head molding during labor reached about 9 percent under the pressure exerted by the birth canal and pelvic floor muscles. That molding reduced the reaction forces on the pelvic floor by roughly 19 percent and decreased muscle stretching by about 2.6 percent, suggesting that adequate molding can help protect the mother from injury.11PubMed. A numerical study on fetal head molding during labor
Why Position Matters So Much
If the pelvis were a rigid ring, position would be irrelevant. Because the pelvis reshapes dynamically, the position of the mother’s body during labor directly changes how much space is available. This is one of the most practically important and underappreciated facts about childbirth.
MRI-based pelvimetry has shown that upright positions significantly expand pelvic bony dimensions. In squatting and hands-and-knees positions, the sagittal outlet increased compared to lying on the back, and the interspinous diameter, a critical measurement at the narrowest part of the mid-pelvis, widened by about 7 millimeters in upright positions.12PubMed. MR obstetric pelvimetry: effect of birthing position on pelvic bony dimensions A study using dynamic external pelvimetry in third-trimester pregnant women confirmed that shifting positions modified multiple pelvic diameters simultaneously: the transverse diameter of the pelvic outlet widened, the tilt of the inlet changed to facilitate fetal descent, and the distance between the ischial tuberosities (the sit bones) increased.13PubMed Central. Dynamic External Pelvimetry Test in Third Trimester Pregnant Women: Shifting Positions Affect Pelvic Biomechanics and Create More Room in Obstetric Diameters
A computational simulation comparing squatting under pregnant and non-pregnant conditions found that the pelvic outlet expanded more in the pregnant simulation. The anteroposterior outlet diameter increased by about 6.1 millimeters and the transverse diameter by about 11 millimeters during squatting under pregnant conditions, versus only 4.1 and 2.6 millimeters under non-pregnant conditions. Peak increases occurred during the dynamic movement of squatting rather than in the final resting position, meaning the act of moving may matter as much as the position itself.14PubMed. The effects of squatting while pregnant on pelvic dimensions: A computational simulation to understand childbirth
Even simpler changes help. An MRI-based reconstruction study found that just flexing the hips and knees increased the anteroposterior diameter of the pelvic outlet by an average of 4 millimeters and reduced pelvic inclination by 25 degrees compared to lying flat on the back.15PubMed Central. Impact of supine vs. hip- and knee-flexed position on anteroposterior diameter and inclination of the female pelvic outlet: an MRI-based 3D reconstruction study Regular squatting has been associated with improved sacroiliac flexion and a larger pelvic outlet, facilitating the passage of the baby.16PubMed Central. Squatting, pelvic morphology and a reconsideration of childbirth difficulties
The evidence is not entirely one-directional, though. A systematic review of laboring people’s mobility under low-dose epidural analgesia found that the relationship between position and outcomes is complicated by factors like the stage of labor and the type of lying-down position used. Lateral decubitus (lying on the side), for example, allows free movement of the sacrum and has been shown to be beneficial during the second stage of labor, even though it is technically a “lying down” position.17PubMed Central. Influence of Laboring People’s Mobility and Positional Changes on Birth Outcomes in Low‐Dose Epidural Analgesia Labor: A Systematic Review with Meta‐Analysis The lesson is not that one position is universally best. It is that immobility, especially flat-on-the-back immobility, restricts a process that is inherently dynamic.
When the Widening Goes Too Far
The normal widening at the pubic symphysis during delivery can occasionally become excessive. Pubic symphysis diastasis is a rare complication in which the joint separates beyond its normal range, sometimes with an audible crack. The typical presentation involves severe pain with any weight-bearing activity like walking or climbing stairs, making movement in the postpartum period extremely difficult. In severe cases, it can be accompanied by sacroiliac joint disruption or urinary incontinence.18PubMed Central. Pubic Symphysis Diastasis: A Case Series and Literature Review There is no standardized treatment protocol; management ranges from pelvic binding and rest to surgical intervention depending on severity.4PubMed Central. Peripartum Pubic Symphysis Diastasis-Practical Guidelines
Birth Canal Shape Varies More Than You Might Think
Discussions of pelvic opening often treat the pelvis as a standard-issue structure, but the human birth canal is highly variable in shape, both within and among populations, and differences in pelvic shapes have been associated with differences in the mechanism of labor.19PubMed Central. Shaping birth: variation in the birth canal and the importance of inclusive obstetric care Some women have a rounder inlet, others a more oval one. The relative proportions of the inlet, mid-pelvis, and outlet differ from person to person. This variation means that the same fetal head size may pass easily through one pelvis but encounter difficulty in another, and that the optimal position and mechanism of rotation for the baby during labor also differ.
This variability is part of why clinical predictions of “adequate” or “inadequate” pelvis size are unreliable. A pelvis that looks borderline on imaging may function perfectly well during labor because the dynamic changes described above create room that a static measurement cannot capture. Pelvimetry, the measurement of pelvic dimensions, has largely fallen out of favor as a routine screening tool for this reason.
Why the Human Pelvis Does Not Simply Evolve to Be Wider
If birth is so tight, why hasn’t evolution just given humans a bigger pelvis? The classic explanation, sometimes called the obstetrical dilemma, proposed that the pelvis is a compromise between the need for a wide birth canal and the need for efficient walking on two legs.20PubMed Central. Developmental evidence for obstetric adaptation of the human female pelvis Recent work has challenged parts of this story. Biomechanical studies indicate that pelvic width does not actually affect the efficiency of bipedal walking very much, so the locomotion constraint may be less important than once thought. Instead, bipedalism may have primarily constrained the flexibility of the pubic symphysis. In most mammals with large fetuses, the pubic symphysis opens much wider during birth than it does in humans. Being upright and bearing the weight of pregnancy for months may require a firmer, less flexible pubic joint. A narrow pelvis also provides better support for the weight of the internal organs and the large fetus during the long human gestation period.21PubMed Central. Evolution of the human pelvis and obstructed labor: new explanations of an old obstetrical dilemma.
The selective pressures shaping the pelvis have been multiple and sometimes conflicting throughout human evolutionary history, involving not just locomotion and childbirth but also thermoregulation.22PubMed Central. The evolution of the human pelvis: changing adaptations to bipedalism, obstetrics and thermoregulation Meanwhile, other researchers have argued that the length of human pregnancy is constrained not by pelvic size at all but by limits on maternal metabolism: the mother’s body can only sustain fetal energy demands up to a point before it runs out of capacity, regardless of pelvic dimensions.23PubMed Central. Metabolic hypothesis for human altriciality The honest state of the science is that the obstetrical dilemma is probably real but more complicated than the textbook version suggests.
The Human Pubic Symphysis as an Evolutionary Solution
One clue to how evolution has dealt with tight childbirth comes from comparing humans to other primates. A study evaluating pubic symphysis fusion across primate species found that most adult primates eventually fuse this joint permanently. In humans, the symphysis remains open throughout life. While delayed or absent fusion is more common in female primates than in males, suggesting an obstetric role, the relationship is not perfectly clean: some species with a tight fit between fetal head and birth canal still fuse the symphysis. The preservation of an unfused symphysis throughout life in humans is best interpreted as a convergent evolutionary adaptation driven by the demands of childbirth.24PubMed Central. Variation in Pubic Symphysis Fusion Across Primates: Implications for Obstetric Adaptation In other words, our never-fusing pubic joint is itself an obstetric feature, a permanent concession to the need for the pelvis to give a little during birth.
Prenatal Exercise and Pelvic Floor Outcomes
A common question is whether anything done before labor can change how the pelvis performs during delivery. A scoping review of pelvic mobility exercises found that activities targeting joint mobility, standing posture, and movement freedom aim to optimize the physiology of childbirth by increasing pelvic diameter and sacrum mobility, and facilitating fetal descent.25Revista Gaúcha de Enfermagem. Pelvic-joint and muscle mobility exercises in preparing for childbirth: a scoping review
A randomized controlled trial tested a supervised prenatal exercise program and found that it did not change the duration of the second stage of labor or the mode of delivery. But at six months postpartum, women in the exercise group had a substantially lower rate of levator ani muscle avulsion compared to the control group, roughly 21 percent versus 53 percent.26PubMed Central. Pelvic Floor Adaptation to a Prenatal Exercise Program: Does It Affect Labor Outcomes or Levator Ani Muscle Injury? A Randomized Controlled Trial In other words, exercise did not make the pelvis open wider, but it appeared to make the pelvic floor muscles better at surviving the stretch.
Postpartum Recovery of the Pelvis
After delivery, the hormonal loosening gradually reverses. Relaxin levels drop, collagen begins rebuilding in its original density, and the pelvic joints tighten back toward their pre-pregnancy configuration. A study comparing early postpartum women to women who had never given birth found that by about three months after delivery, pelvic stability and muscle activation patterns during walking were similar to those of the control group.27PubMed. Lower extremity kinetics and muscle activation patterns during gait in early postpartum women: 42-day and 3-month comparison with nulliparous controls The bony pelvis does not permanently widen in most cases. It returns to something close to its original dimensions, though the soft tissue may take longer to fully recover, and some degree of pelvic floor laxity can persist, particularly after multiple vaginal deliveries.
Symphysiotomy as a Surgical Alternative
The principle that the pubic symphysis can be separated to create more room was, at various points in medical history, applied surgically. A symphysiotomy is an operation in which the fibers of the pubic symphysis are partially divided to allow separation of the joint and enlargement of the pelvic dimensions during childbirth.28PubMed Central. Symphysiotomy for feto-pelvic disproportion This procedure requires far fewer resources than a cesarean section and was historically used in settings where surgical delivery was unavailable.29PubMed. Symphysiotomy for obstructed labour: a systematic review and meta-analysis It has largely been replaced by cesarean delivery in most of the world, but its existence underscores the basic mechanical principle at work: the pelvis can and does open at its joints, and even a small amount of additional separation at the symphysis can make the difference between an obstructed labor and a successful one.