Pediatrics · Part One · Before Birth and Birth
Lesson 06 / 57
The Pelvis and Room to Grow: What Pregnancy Changes in the Pelvic Ring
About 20 percent of pregnant women have pelvic girdle pain at any moment. What separates them from women without pain is not how loose the joints are. It is whether both sides are loose by the same amount.
The maternal pelvis is a three-piece ring. Two innominate bones meet the sacrum at the sacroiliac joints and each other at the pubic symphysis. About 20 percent of pregnant women have pelvic girdle pain at any moment, and about 45 percent report pelvic or low back pain across a pregnancy. Measured joint laxity does not separate them. Left to right asymmetry does, at 37 percent against 4 percent. The Unified Model of Tone reads the pelvis as a loaded ring whose symmetry is the informative measurement.
Point prevalence of pelvic girdle pain in pregnancy
about 20 percent
Serum relaxin
peaks at 12 weeks, then declines toward term
Asymmetric sacroiliac laxity, pain against no pain
37 percent against 4 percent
Chiropractic care for one-sided pelvic girdle pain
no significant difference from usual care in 56 women
Pelvic girdle pain, defined
Pain arising around the sacroiliac joints and the pubic symphysis. It is a distinct entity from pregnancy-related low back pain, although the two often occur together and are usually reported under one label. European guidelines diagnose it with pain provocation tests and the active straight leg raise, not with imaging.
How the ring carries load
Body weight passes from the spine into the sacrum, across the two sacroiliac joints and into the hips. Dense ligaments hold those joints and muscle crosses them. Stability comes from the fit of the joint surfaces and from the compression applied across them.
01The ring itself
The pelvis is three bones and three joints, and its shape varies between women
The pelvis is a ring assembled from separate pieces. Two innominate bones form the sides and front, and the sacrum wedges between them at the back. They meet at three joints: a sacroiliac joint on each side, and the pubic symphysis in front. Each innominate is itself the ilium, ischium and pubis, fused during childhood.
The innominates and the sacrum meet at joints rather than fusing to one another, and that is the whole reason pregnancy can change how the ring behaves. Dense ligament holds each joint and muscle crosses it, so the ring is a tensioned structure rather than a rigid hoop.
Straps that listen
The ligaments of the pelvis are not passive webbing. The round ligament carries smooth muscle continuous with the wall of the uterus, and the uterosacral ligaments carry autonomic fibers along their course. Tension in the pelvic ring and tension in the uterus draw on the same supply.
Ligamentous tension is therefore a regulated state rather than a fixed length. A strap under uneven pull biases the room available on one side, and that pull is set centrally as much as locally.
Shape is not standard
The birth canal is usually described as though there were one shape. There is not. A study of canal shape across human populations found that women are extremely variable in the shape of the bony birth canal, with different populations having differently shaped canals (Betti 2018). Neutral genetic drift and differential migration explain most of that pattern.
The same paper reports that the canal fits the neonate tightly, which is not the case in other apes. Modern obstetric understanding was developed largely on studies of European women.
02Findings
What the research shows
The figures below come from a European guideline, three systematic reviews, a hormone study, a measurement study and a randomized trial.
03How common, and why
Pelvic girdle pain is common, and the hormone usually blamed for it does not explain who hurts
Pelvic girdle pain reaches about 20 percent of pregnant women at any moment (Vleeming 2008). Across a whole pregnancy the figure is about 45 percent, with severe disability in about 8 percent (Wu 2004). Two explanations are offered. The first is hormonal: relaxin softens the ligaments, the joints loosen, pain follows. The second is mechanical: the two sides stop matching.
What raises the risk, and what does not
The guideline names two risk factors: a history of previous low back pain, and previous trauma to the pelvis. The list of non-risk factors is as useful. It records agreement that contraceptive pills, the interval since the last pregnancy, height, weight and smoking are not risk factors, with age in the same list. A woman who develops this pain did not bring it on through any of those. Wu and colleagues leave open what mechanism produces the disability, and the next two findings narrow it.
The relaxin hypothesis did not survive review
Start with the timing, which is normally stated backwards. Serial measurement in 40 pregnant women found relaxin rose to a peak at 12 weeks, then gradually declined toward term (Anumba 2009). Week 12 is where the curve turns, not where it starts.
A systematic review then screened 731 references and included six studies, four rated high quality. Among those four, three found no association between pregnancy-related pelvic girdle pain and relaxin levels (Aldabe 2012). The review graded the evidence as low. Relaxin rises and falls in every pregnancy. Pain does not.
The measurement that separated the groups
Damen and colleagues measured sacroiliac laxity by Doppler imaging of vibrations in 163 women at 36 weeks (Damen 2001). Mean laxity was 3.0 threshold units in the 73 women with moderate or severe pain and 3.4 in the 90 without.
The left to right difference told another story, averaging 2.2 threshold units against 0.9. Within the painful group, women with asymmetric laxity scored higher on pain, on the provocation test and on the disability scale. Increased laxity is not associated with this pain. The difference between sides is.
04What helps the pain
Exercise has the best evidence, and manual therapy does not beat sham
Everything in this section is pain evidence, measured against pain outcomes in pregnant women.
A Cochrane review included 34 randomized trials of 5,121 pregnant women, from 12 to 38 weeks of gestation (Liddle 2015). Land-based exercise reduced low back pain, standardized mean difference -0.64, in 645 participants across seven studies. An eight to twelve week program reduced the number reporting low back and pelvic pain, risk ratio 0.66. Sick leave fell as well, risk ratio 0.76.
The same review carries a null inside it. For pelvic pain, group exercise added to information about managing pain produced no significant difference against usual prenatal care, risk ratio 0.97. The strong exercise result belongs to low back pain. Reported adverse effects were minor and transient.
Manual therapy, measured against sham
A meta-analysis included 10 studies on 1,198 pregnant women, predominantly massage and osteopathic manipulative therapy (Hall 2016). It found positive effects on pain intensity against usual care and relaxation, but not against sham interventions. The authors conclude that there is limited evidence to support these therapies during pregnancy.
The trial that tested chiropractic
Fifty-six women with dominating one-sided pelvic girdle pain were randomized, 28 to individualized chiropractic treatment and 28 to conventional primary health care (Gausel 2017). There was no significant difference in sick leave, pelvic pain, disability or general health status during pregnancy or after delivery. The confidence intervals were wide.
The guideline recommends adequate information, reassurance, and individualized exercises (Vleeming 2008). It recommends against mobility palpation tests, imaging and diagnostic injections for making the diagnosis.
Pain scores are one reading of the ring. How evenly the two sides share a load is another, and it is the reading hands-on care is aimed at.
05Constraint before birth
Intrauterine constraint is documented, and it is not something a mother caused
The cleanest evidence that intrauterine position shapes a newborn comes from twins. Littlefield and colleagues obtained medical histories on 140 sets of twins, 280 infants, made up of 46 concordant and 94 discordant pairs (Littlefield 2002). Twin pairs share one uterus, one pregnancy and one mother. What differs between them is position.
The lower twin in utero was significantly more likely to be affected by deformational plagiocephaly. The more severely affected infant was more likely to have neck involvement, meaning torticollis or neck tightness, and to have been carried in a vertex position. Neither sleeping position nor sex was associated. The authors read this as strong support for an in utero cause.
What that does not mean
It does not mean a mother did anything wrong. Position inside a shared uterus is not a choice, and the twin design makes that plain: two infants, identical maternal circumstances, different outcomes. Nothing here supports the idea that a mother caused, or could have prevented, her child’s head shape or neck asymmetry.
What the first year does with it
What the womb sets, early movement refines. A strong turning preference, a hip that clicks or limits, and a flattening on one side of the skull are all worth catching in the first weeks. The first year is when a body reworks what position established.
Care at that stage is light, specific and graded to a small body, and it runs alongside the well child checks that track range of motion month by month. Head shape and torticollis covers what happens after birth, and tummy time and the curve covers the movement that does the reworking.
06At the pelvis
Care in pregnancy attends to how evenly the ring shares its load
The examination comes first. Where the sacrum sits between the innominates, how each sacroiliac joint answers a load applied across it, and whether the symphysis is tender are all read by hand. The deep lateral rotators of the hip frame the lower ring and report the same thing from the muscular side.
Those rotators are the piriformis, the gemelli, the obturators and quadratus femoris. When one sacroiliac joint drifts past its firm range, the muscles around the ring and up the back tighten to hold it, which is why every position eventually feels wrong and none of them lasts.
The contact
What follows is sustained light contact, specific to the joint that has stopped answering and graded to a pregnant body. There is no twist and no thrust. A woman lies on her side or is supported in front, and the same contact that reads the tissue is the contact that works on it.
The aim is stated in the same terms as the measurement. In 163 women at 36 weeks, the left to right difference in sacroiliac laxity averaged 2.2 threshold units in those with moderate or severe pain and 0.9 in those without (Damen 2001). Evenness between the sides is the variable.
Why the model attends here
A ring that shares its load unevenly is holding a compensation, and holding costs. The most talkative tissue in such a pelvis is frequently the one splinting the pattern rather than the one that started it. That is why the examination covers the whole ring before the hands settle anywhere.
The model expects an input matched to that pattern to show up first in how evenly the ring answers a load, and only afterward in what a woman reports. Assessment is where the accuracy lives, and the contact that follows it is the easy half.
07The model’s claim
What the Unified Model of Tone predicts about the pregnant pelvis
Everything above is established science, including trial results that came back flat. What follows is this model’s reading of it, stated as ours rather than drawn from the papers cited.
The model treats the pelvic tension network as the body’s geometric self-registration. The network does not report the ring’s shape to some reader elsewhere. Its present organization is that shape. The Damen result already has this form. Mean laxity of 3.0 against 3.4 threshold units separated nobody, while asymmetry at 37 percent against 4 percent separated the groups cleanly (Damen 2001).
The uterus hangs from that ring by its ligaments. When tension across the ring is uneven, the uterus suspended from it takes an uneven shape, and the space available to the fetus contracts in predictable directions.
The prediction
From that follows a claim the pain literature does not make. Asymmetric load sharing in the pelvic ring is not a local finding, because a change in registered geometry becomes information everywhere. The model predicts that measurable differences in postural control, autonomic variability and sleep continuity are more likely in the same women, while compensation can hold any one of those readings steady. Those are maternal readings of reflex responsiveness, variability structure and recovery time, three faces of the one variable the model reads.
The same geometry carries a second claim. Reducing asymmetric pelvic tension should expand the space available to the fetus. A fetus will often take up that room when it opens, because the structure around it now permits a configuration it did not permit before.
The constraint mechanism carries its own test. Measure pelvic asymmetry before and after an input directed at it, with the assessor never shown the fetal position, and read the change in asymmetry as the mediator rather than the outcome. Repositioning that tracks the measured change in asymmetry confirms the mechanism. The model stakes the mechanism, and outcome figures belong to trials that have not yet been run.
This is a claim about how pregnancy mechanics are organized rather than about what treatment does. If sacroiliac symmetry, postural control, autonomic variability and sleep continuity are shown to move together in the same women, the unification claim is confirmed.
08The tone reading
How the pregnant pelvis expresses tone
Every topic in this library expresses all of tone. In the pregnant pelvis three aspects carry the signature, because a ring that carries the whole body across two joints is defined by how evenly the load is shared.
Constraint
A three-piece ring bounds how the load can travel. The birth canal fits the neonate tightly, so geometry sets the limits before movement does.
Load
Body weight crosses the sacroiliac joints on its way to the legs. Pregnancy adds mass and moves the center of gravity forward every week.
Time course
Relaxin peaks at 12 weeks and falls toward term, while pain typically increases later. The hormone curve and the symptom curve do not match.
The remaining foundations run through this topic as well. Coupling: the two sacroiliac joints and the symphysis cannot move independently. Gain: how loudly a joint reports strain is set centrally, not at the ligament. Set point: a body carrying pain all day holds a different resting arousal. Prediction: rising from a chair is planned before the load arrives. Input quality: a provocation test works because of where the load is applied. Oscillation: walking is a rhythm, and an asymmetric ring alters every step. These are readings of one organization rather than separate systems, which is the core claim of the Unified Model of Tone.
09Across the library
How this page relates to the rest of the library
The maternal pelvis sits between pregnancy physiology and what a newborn examination finds.
Where claims about how a baby lies are examined, and what evidence stands behind them.
Maternal autonomic state in pregnancy, read as a measurable variable.
What intrauterine constraint leaves behind after birth, and what the helmet trial found.
The postnatal side of head shape, added to position before birth.
The safety record for pediatric care, reported separately from efficacy.
Mechanical and physiological demand as a measurable state, in adult detail.
Movement as a measurable input, and what controlled trials of exercise report.
10Frequently asked
Questions families ask about the pelvis in pregnancy
How common is pelvic girdle pain in pregnancy?
About 20 percent of pregnant women have pelvic girdle pain at any given point. Across a whole pregnancy the figure is higher: roughly 45 percent of pregnant women and 25 percent postpartum report pelvic girdle pain, low back pain or both. Average pain intensity during pregnancy is 50 mm on a visual analog scale. Serious pain occurs in about 25 percent of patients and severe disability in about 8 percent. After pregnancy, problems remain serious in about 7 percent.
Does relaxin cause pelvic pain in pregnancy?
The evidence for that is weak. A systematic review screened 731 references, included six studies, and rated four of them high quality. Among those four, three found no association between pregnancy-related pelvic girdle pain and relaxin levels, and the review graded the evidence as low. The timing is also usually stated backwards. Serial measurement shows relaxin rising to a peak at 12 weeks, then declining gradually toward term, so it falls through the months when pain typically increases.
Why does my pelvis hurt more on one side?
Because asymmetry, rather than looseness, is what separates women with pelvic pain from women without it. In 163 women measured at 36 weeks, mean sacroiliac laxity was 3.0 threshold units in those with moderate or severe pain and 3.4 in those with none or mild, which is not a significant difference. The left to right difference was 2.2 against 0.9 threshold units. Asymmetric laxity was present in 37 percent of the painful group and 4 percent of the comparison group.
What actually helps pregnancy-related pelvic and low back pain?
Exercise has the best evidence, and it is stronger for low back pain than for pelvic pain. A Cochrane review of 34 trials and 5,121 pregnant women found land-based exercise reduced low back pain, standardized mean difference -0.64. An eight to twelve week program reduced the number of women reporting low back and pelvic pain, risk ratio 0.66, and cut sick leave. For pelvic pain alone, group exercise showed no significant difference against usual prenatal care, risk ratio 0.97. Guidelines recommend information, reassurance and individualized exercise.
Can care during pregnancy make more room for my baby?
That is the model’s claim, and it is a claim about geometry. The uterus hangs from the pelvic ring by its ligaments, so a ring that is tensioned unevenly gives the uterus an uneven shape and narrows the space along predictable lines. Easing that asymmetry should open the room again, and the measurement that would confirm it is the change in pelvic asymmetry itself. How your baby is lying, and how your baby is born, are read and decided by the obstetric team who can see the presentation directly.
My baby was born with a flat spot and a tight neck. Did I cause it?
No. Intrauterine position is not something a mother chooses, and the clearest evidence comes from twins. In 140 twin sets, 280 infants, the lower twin in utero was significantly more likely to develop deformational plagiocephaly, and the more severely affected infant was more likely to have neck involvement. Both infants shared one uterus, one pregnancy and one mother, so position was what differed. Sleeping position and sex were not associated. What follows from a finding like that is examination and referral, rather than blame.
What happens at a pelvic balancing visit in pregnancy?
It starts with an examination. Your chiropractor reads where the sacrum sits between the innominate bones, how each sacroiliac joint answers a load, and how the deep hip rotators are sharing that load across the ring. Where one side has quietly tightened, the contact is sustained and light, specific to that joint, with no twist and no thrust. You lie on your side or supported in front, whichever is comfortable. Most women describe the visit itself as settling, and evenness between the two sides is the aim throughout.
11The sources
References
10 primary sources, each linked to its PubMed record. Figures quoted on this page were checked against the published abstract.
Related evidence