The Anatomy of Birth: Why the Baby Must Turn

The passage: a bent tube of bone

Obstetricians speak of three P's — the passage, the passenger and the powers — and the passage comes first because it cannot change. The bony canal is not a cylinder but a short, curved passage with three levels. At the top is the INLET (pelvic brim), bounded by the sacral promontory behind, the arcuate and pectineal lines at the sides and the pubic symphysis in front; its longest measurement runs TRANSVERSELY, from side to side. In the middle is the MIDCAVITY, and its narrowest measurement of all is the INTERSPINOUS diameter between the two ischial spines — the true bottleneck of the whole canal, and the reason those spines are the landmark for everything. At the bottom is the OUTLET, bounded by the pubic arch in front, the ischial tuberosities at the sides and the coccyx behind; here the longest measurement runs ANTEROPOSTERIORLY, from front to back. The exact figures, the obstetric conjugate and the four classical pelvic shapes are set out in the pelvic cavity and the obstetric pelvis; what matters here is only the ninety-degree twist between the top opening and the bottom one. Add to that the fact that the canal is BENT: it runs downwards and backwards from the inlet, then turns sharply downwards and FORWARDS at the level of the pelvic floor, so its axis traces a J-shaped curve — the CURVE OF CARUS. The baby is not dropped through a chimney. It is steered around a bend.

Lining that bony ring is the SOFT passage, and it is the part that actually gives way. During pregnancy the uterus differentiates into two functionally opposite halves: a thick, muscular UPPER SEGMENT that contracts and shortens, and a thin, passive LOWER UTERINE SEGMENT that is drawn upwards and stretched — the part a caesarean incision is deliberately placed in, because it is thin, relatively bloodless and heals well. Below it the CERVIX, a collar of dense collagen with only a little smooth muscle, must be remodelled entirely: enzymes break down its collagen bundles so that it softens and "ripens" before it can be pulled open. Below that lie the vagina, whose rugae unfold like a concertina, and finally the muscular shelf of the pelvic floor and the skin and muscles of the perineum — the last two centimetres, and the ones that tear.

The passenger: a skull built to be squeezed

The fetal skull is the only part of the body that is deliberately unfinished at birth. The vault is made of separate plates — two frontal, two parietal, two temporal and the occipital — joined not by fused bone but by membranous seams. The SUTURES are those seams: the SAGITTAL suture running front to back between the two parietal bones; the CORONAL suture running across, between the frontal and parietal bones; the LAMBDOID suture running across behind, between the parietals and the occipital; and the FRONTAL (metopic) suture running forwards between the two frontal bones. Where several sutures meet, the membrane widens into a FONTANELLE. Two are palpable and they are told apart by shape, not by size: the ANTERIOR fontanelle (bregma) is DIAMOND-shaped, four sutures meeting, and closes at around eighteen months; the POSTERIOR fontanelle (lambda) is TRIANGULAR, three sutures meeting, and closes within a few weeks. Because the plates are only tethered, they can slide and overlap under pressure — MOULDING — so that the head narrows in one diameter and lengthens in another as it passes, and re-expands over the days after birth. But the sutures do a second job that matters just as much on the night: they are BRAILLE. A finger that finds a diamond in front knows the occiput is at the back; a finger that finds a triangle in front knows the baby has turned face-upwards. Position is determined by feel, in the dark, from these landmarks alone.

How big the head is depends entirely on how it is held. This is the single most useful idea in obstetric anatomy: the fetal head does not have one size, it has a size for every ATTITUDE — every degree of flexion or extension of the neck. In full FLEXION, chin pressed on chest, the diameter presented to the pelvis is the SUBOCCIPITOBREGMATIC, about 9.5 cm, measured from below the occipital bone to the centre of the anterior fontanelle. This is the smallest diameter the head can offer, and the presentation is called a VERTEX presentation. Let the head deflex a little so the neck is neutral, and what presents is the OCCIPITOFRONTAL diameter, about 11.5 cm, from the occiput to the root of the nose — two whole centimetres wider, in a canal that has none to spare. Extend the neck halfway, so the brow leads, and what presents is the MENTOVERTICAL diameter of about 13 cm, from the chin to the top of the vault: the LARGEST diameter of the fetal head, in a BROW presentation, and it will not pass an average pelvis at term — brow presentation is usually obstructed labour and needs a caesarean. Extend the neck completely, chin first, and the head shortens again to the submentobregmatic in a face presentation. The lesson is stark. Nothing about the baby's head has grown or shrunk; the difference between an easy birth and an obstructed one can be four centimetres of geometry created by the angle of a neck. FLEXION IS WHAT MAKES THE SMALLEST DIAMETER PRESENT.

The powers: a muscle that pulls itself shorter

Uterine contractions are not like any other muscular contraction in the body. They begin at PACEMAKERS near the cornua and sweep downwards over the uterus in a wave — the FUNDAL DOMINANCE that makes the fundus contract hardest and longest, so the net force pushes the fetus down and out rather than squeezing it in place. And when the upper segment relaxes, it does not return to its former length: it RETRACTS, keeping some of the shortening it just achieved, so that with each contraction the upper segment becomes progressively thicker and shorter while the lower segment and cervix are pulled progressively thinner and longer. The visible result at the cervix comes in two stages. EFFACEMENT is the cervix being taken up into the lower segment until its canal disappears and it becomes a paper-thin rim — measured as a percentage. DILATATION is that rim then being pulled open from a closed os to ten centimetres. In the first labour effacement largely precedes dilatation; in later labours the two happen together. Only when dilatation is complete does the second stage begin, and a second power joins in: maternal expulsive effort — a Valsalva against a closed glottis with the diaphragm and abdominal wall driving down on the uterus — a voluntary force that is added to an involuntary one, and which the woman feels as an irresistible reflex urge the moment the presenting part presses on the pelvic floor and rectum.

The seven cardinal movements

These are not seven separate manoeuvres. They are one continuous journey, described at seven moments. ONE — ENGAGEMENT. The widest transverse diameter of the head, the biparietal, passes below the plane of the inlet. Because the inlet is widest from side to side, the head enters with its sagittal suture running TRANSVERSELY: the baby faces the mother's left or right hip, not her back. Clinically the head is engaged when no more than two fifths of it can still be felt above the pubic symphysis. TWO — DESCENT. Driven by contractions, by the retraction of the upper segment and later by maternal effort, the head travels down the axis of the canal. It is measured as STATION: the level of the leading bony point in centimetres above (minus) or below (plus) the ischial spines, with station zero meaning the head has reached the spines — the narrowest level. THREE — FLEXION. As the descending head meets the resistance of the cervix, the pelvic side walls and above all the pelvic floor, the neck flexes further, the chin is driven onto the chest, and the presented diameter swaps from occipitofrontal (11.5 cm) to suboccipitobregmatic (9.5 cm). The pelvis has, in effect, folded the passenger smaller. FOUR — INTERNAL ROTATION. The occiput, now the leading point, strikes the forward-sloping gutter of the levator ani and turns ninety degrees anteriorly, so that the sagittal suture rotates from the transverse into the ANTEROPOSTERIOR diameter and the occiput comes to lie beneath the pubic arch. This is the movement the whole of human birth is built around: the head entered a transverse door and must leave through an anteroposterior one. It happens at the level of the ischial spines, on the pelvic floor, and it is soft tissue — not bone — that performs it.

FIVE — EXTENSION. The canal now turns forwards, and so must the head. The nape of the neck pivots against the lower border of the pubic symphysis, and the head EXTENDS around that fixed point, sweeping up and forwards along the curve of Carus. In order, the occiput, then the bregma, brow, nose, mouth and chin are delivered over the stretched perineum. The single commonest reason a midwife's hand rests on the advancing head is to keep that extension SLOW and controlled — a head that extends suddenly tears everything below it. SIX — RESTITUTION and EXTERNAL ROTATION. The moment the head is free it untwists: RESTITUTION is the passive forty-five degree turn that undoes the twist internal rotation put into the neck, realigning the head with the shoulders. EXTERNAL rotation follows and is not passive at all — it is the visible sign that the SHOULDERS are themselves doing internal rotation deeper in the pelvis, turning their bisacromial diameter from the transverse into the anteroposterior so that they too can pass the outlet. The head, already outside, is dragged round with them. SEVEN — EXPULSION. With the shoulders now anteroposterior, the ANTERIOR shoulder appears under the pubic arch and is delivered first with gentle downward traction; the POSTERIOR shoulder then sweeps over the perineum with upward traction, and the rest of the body follows without resistance. Seven movements, one bend, and not a single one of them optional.

The pelvic floor in labour — and where it tears

The pelvic floor has to do two contradictory things on the same night: steer, and then get out of the way. For most of the second stage the levator ani is an active guide: its funnel shape directs internal rotation, and the puborectalis sling holds the anorectal angle. Then it must be displaced. The muscle is drawn upwards and thinned, the sling is pushed back, and the vaginal opening is stretched from a slit into a ring wide enough for a 9.5 cm head. In front of the anus the PERINEAL BODY — the fibromuscular node where bulbospongiosus, the superficial and deep transverse perineal muscles, the external anal sphincter and levator ani all converge — is flattened and stretched to several times its resting length. It is the keystone of the perineum, and it is precisely here that the tissue fails. PERINEAL TEARS are graded by exactly what they involve. FIRST DEGREE: skin of the perineum and vaginal mucosa only, no muscle. SECOND DEGREE: through the perineal muscles and the perineal body, but the anal sphincter is intact — this is also the depth of a standard episiotomy. THIRD DEGREE: the tear reaches the ANAL SPHINCTER COMPLEX, and is subdivided by how much — 3a, less than half the thickness of the external anal sphincter torn; 3b, more than half the external sphincter torn; 3c, the INTERNAL anal sphincter torn as well. FOURTH DEGREE: through the sphincter complex and into the anal mucosa itself. Third- and fourth-degree tears are grouped as obstetric anal sphincter injuries and are repaired formally in theatre, because the internal sphincter is the smooth muscle responsible for resting continence and the external sphincter for the voluntary squeeze — an unrecognised or poorly repaired tear is a leading cause of long-term FAECAL INCONTINENCE and urgency in otherwise healthy young women. The anatomy of that two-sphincter mechanism is set out in the anal canal.

Two stages, two pains, two blocks

Labour pain changes nerve supply halfway through, and every method of relieving it depends on knowing when. FIRST-STAGE pain comes from the uterine body and the dilating cervix: visceral afferents that travel with the sympathetic fibres through the hypogastric plexuses to enter the spinal cord at T10–L1. Because that input is visceral and enters over several segments, the pain is diffuse, cramping and referred to the lower abdomen, the lower back and sometimes the upper thighs — the woman cannot point to it. To abolish it you must block T10–L1: a lumbar EPIDURAL is the standard, a spinal or a paravertebral block will also do it. A pudendal block will not — it is aimed at entirely the wrong segments, and this is one of the most reliably examined points in the whole subject. SECOND-STAGE pain is different in kind. Now the presenting part is distending the vagina, the pelvic floor and the perineum, and that is SOMATIC pain, sharp and precisely localised, carried by the PUDENDAL nerve from S2, S3 and S4 — as traced in the pudendal nerve. It can be abolished by infiltrating local anaesthetic where that nerve is at its most accessible: as it hooks around the sacrospinous ligament just medial and behind the ISCHIAL SPINE, a landmark the operator can feel through the vaginal wall. A pudendal block therefore numbs the perineum for delivery, for an episiotomy and its repair, and for a low forceps — but it does nothing at all for the contractions themselves, because those are still arriving at T10–L1.

When the geometry fails

CEPHALOPELVIC DISPROPORTION is the plain statement that this head will not pass this pelvis — sometimes because the head is large or deflexed, sometimes because the pelvis is contracted or android in shape, and it is diagnosed by failure of descent despite good contractions, not by measurement in advance. OCCIPITOPOSTERIOR position, where the occiput rotates backwards instead of forwards, presents a larger diameter, deflexes the head, and gives a long labour with severe back pain and an early urge to push before full dilatation; many still rotate and deliver, some deliver face-to-pubis, and some need rotational assistance. SHOULDER DYSTOCIA is a purely anatomical emergency: the head is born but the anterior shoulder is caught behind the pubic symphysis. The first response, the McROBERTS manoeuvre, involves no traction at all — the mother's hips are hyperflexed onto her abdomen, which rotates the symphysis upwards and FLATTENS the sacral promontory, straightening the lumbosacral angle and effectively increasing the useful anteroposterior dimension of the inlet so the trapped shoulder slips free. Working the other way, the hormone RELAXIN and the oestrogens of pregnancy soften the ligaments of the pubic symphysis and the sacroiliac joints, so the pelvic ring gains a few millimetres of give — a genuine help in labour, and the reason for the waddling gait, the pubic pain and the sacroiliac ache of late pregnancy. When the powers or the passage fail at the last, INSTRUMENTAL DELIVERY substitutes for them: forceps grip the fetal head across the malar bones and can also rotate it, while the ventouse works by traction on a cup applied over the flexion point, which encourages flexion and therefore the smaller diameter. And when it is all over, one last piece of anatomy decides whether the mother lives: the placental bed is a raw surface fed by spiral arteries, and the only thing that closes them is the contracted, retracted uterine muscle squeezing them shut — the LIVING LIGATURES. A uterus that fails to contract bleeds catastrophically, which is why the fundus is rubbed up, why a uterotonic is given routinely, and why atony is the first cause considered in postpartum haemorrhage.