The Pelvis in Pregnancy and Labour: Structure, Function and Adaptation
The Pelvis
The human pelvis plays a central role in pregnancy and labour. It forms the bony passage through which a baby is born, while also adapting throughout pregnancy to support the growing uterus. Understanding the anatomy and function of the pelvis can empower women and birthing people to approach labour with greater knowledge, confidence and trust in their bodies.
Anatomy of the Female Pelvis
The pelvis is a ring-shaped bony structure located at the base of the spine, joining the trunk and the legs. It is made up of four main bones:
Two innominate bones (hip bones), each consisting of the ilium, ischium and pubis.
The sacrum, a triangular bone formed by fused vertebrae, connecting the spine to the pelvis.
The coccyx (tailbone), located at the base of the sacrum.
The joints of the pelvis – particularly the sacroiliac joints and the pubic symphysis – allow a small but significant degree of movement. During pregnancy, the hormone relaxin increases the mobility of these joints, making the pelvis more flexible in preparation for birth.
The Pelvic Inlet, Midpelvis and Outlet
The pelvic cavity is not uniform. It is divided into three sections that the baby must navigate during labour:
Pelvic inlet (brim) – The upper opening, shaped more like an oval, where the baby first engages.
Midpelvis – The narrowest section, where the ischial spines are located. The baby must rotate to pass through this level.
Pelvic outlet – The lower opening, widest from front to back, where the baby’s head crowns and the body is born.
Each section requires specific movements from the baby’s head and body – a process known as the mechanisms of labour.
Pelvic Types and Their Influence on Birth
Gynaecologists and anthropologists have historically classified pelvic shapes into four main types:
Gynaecoid – The most common type (seen in ~50% of women), wide and rounded, considered optimal for vaginal birth.
Android – Heart-shaped, narrower, associated with more posterior positions and potentially longer labours.
Anthropoid – Oval-shaped front-to-back, often allowing direct occipito-posterior (back-to-back) births.
Platypelloid – Wide side-to-side and relatively shallow, less common but may affect engagement.
Most people have a mixed pelvic type, and with the support of gravity, mobility, and positioning, the pelvis usually accommodates the baby’s passage.
How the Pelvis Adapts in Pregnancy
Pregnancy hormones – notably relaxin, progesterone and oestrogen – influence the pelvic ligaments and joints:
Pubic symphysis softening widens the pelvis slightly.
Sacroiliac joint mobility allows the sacrum to tilt backwards (sacral nutation) during labour.
Increased lordosis (spinal curve) helps balance the growing bump, shifting posture.
While this adaptation is essential, it can sometimes contribute to pelvic girdle pain (PGP) or symphysis pubis dysfunction (SPD), which affects up to one in five pregnant women in the UK.
➡️ Supportive care can make a big difference. Meeting with our osteopathy partners - One Health - can help relieve pelvic tension, restore mobility, and improve comfort during pregnancy. Many women find regular sessions beneficial in reducing PGP and preparing the pelvis for labour.
The Pelvis in Labour: Biomechanics of Birth
Birth is a dynamic interaction between the mother’s body and the baby. The baby performs a series of cardinal movements – engagement, descent, flexion, internal rotation, extension, restitution and expulsion – while the pelvis accommodates these shifts.
Early labour: The baby’s head enters the pelvic inlet, usually in a sideways position (transverse).
Mid-labour: At the ischial spines, the head rotates to align with the wider pelvic outlet.
Birth of the head: The sacrum moves backwards, and the coccyx flexes, widening the outlet.
Birth of the shoulders and body: The widest diameter of the shoulders passes through as the baby rotates again.
This synchrony illustrates why mobility, upright positions, and trust in the physiology of birth are so important.
Optimising the Pelvis During Labour
Evidence-based strategies can help maximise pelvic space:
Upright positions (squatting, kneeling, standing, hands-and-knees) increase the pelvic outlet diameter by up to 30%.
Asymmetrical movements (such as lunges or stair walking) encourage rotation and descent.
Freedom of movement reduces pain perception and supports oxytocin release.
Birth balls, rebozo techniques and pelvic tilts promote alignment and comfort.
NICE and WHO guidelines both recommend freedom of movement and upright positions as beneficial for labour progress.
Postnatal Recovery: Supporting the Pelvis After Birth
The pelvis continues to adapt after birth as ligaments, joints and muscles gradually stabilise. Some people experience ongoing pelvic or lower back pain, pelvic floor weakness, or abdominal separation.
This is where postnatal rehabilitation is vital. At The Bubba and Me Club, we partner with One Health to offer both a Pregnancy MOT (to support women antenatally) and a Mummy MOT (for postnatal recovery).
Together with osteopathy and holistic postnatal care, these services ensure that women are fully supported before and after childbirth, reducing the risk of long-term complications.
Key Takeaways
The female pelvis is uniquely adapted for pregnancy and childbirth.
Hormonal changes increase pelvic mobility in preparation for labour.
Baby and pelvis work together through a sequence of movements.
Upright, mobile, and supported labours help optimise pelvic dimensions.
Osteopathy, Pregnancy MOT and Mummy MOT provide evidence-based support for pelvic health before and after birth.
Conclusion
The pelvis in pregnancy and labour is not a rigid structure, but a living, adaptable passage designed to bring new life into the world. Science shows that movement, position, and environment can make a profound difference in how effectively the pelvis functions during birth. With the support of our osteopathy team, plus the Pregnancy MOT and Mummy MOT available through our partners at One Health, families can access comprehensive, evidence-based care that supports both antenatal preparation and postnatal recovery.
