Define and describe the diagnosis and management of contracted Pelvis and Cephalopelvic Disproportion.

Pelvic assessment matters only in the context of labour progress.

Definition

A contracted pelvis is one in which one or more diameters are sufficiently reduced to interfere with the normal mechanism of labour. Cephalopelvic disproportion (CPD) is a disparity between the size of the fetal head and the maternal pelvis such that the head cannot safely negotiate the pelvis.

Contracted Pelvis & CPD - Assessment

Pelvic Diameters of the Inlet

• True (anatomical) conjugate — sacral promontory to the inner upper border of the symphysis ≈ 11 cm.
• Obstetric conjugate — sacral promontory to the nearest (prominent) point of the symphysis ≈ 10 cm; the shortest antero-posterior diameter and the one of obstetric importance.
• Diagonal conjugate — lower border of the symphysis to the sacral promontory ≈ 12 cm; the only one measurable clinically (obstetric conjugate = diagonal conjugate − 1.5–2 cm).

Grading (BY the Obstetric Conjugate)

Borderline 9.5–10 cm; moderate 8–9.5 cm; severe < 8 cm. The degree determines whether a trial of labour is safe.

Causes of a Contracted Pelvis

Nutritional/metabolic — rickets and osteomalacia; congenital — Naegele and Robert pelvis, high-assimilation pelvis; and traumatic or disease of the pelvis and spine (fractures, kyphosis, scoliosis, tuberculosis).

Levels of Pelvic Contraction

• Inlet contraction — obstetric conjugate < 10 cm; the head fails to engage, predisposing to malpresentation, non-engagement and cord prolapse.
• Mid-cavity contraction — converging side walls and prominent ischial spines (interspinous < 9.5 cm); causes deep transverse arrest.
• Outlet contraction — a narrow sub-pubic angle and reduced inter-tuberous diameter, often associated with mid-cavity contraction.

Effects on Pregnancy and Labour

During pregnancy — non-engagement of the head, malpresentations (breech, transverse) and a pendulous abdomen. During labour — prolonged labour, early rupture of the membranes and cord prolapse, prolonged/obstructed labour with the risk of uterine rupture and fistula, and fetal asphyxia or birth injury.

Diagnosis

• History — short stature, limp or rickets, and a previous difficult/operative delivery or stillbirth.
• Abdominal examination — a non-engaged head in a primigravida at term, a pendulous abdomen, and malpresentation.
• Pelvic assessment — measurement of the diagonal conjugate and assessment of the sacrum, ischial spines and sub-pubic angle.
• Head-fitting test (Müller–Munro Kerr) — assessing whether the head can be pushed into the brim — and, where indicated, imaging pelvimetry/ultrasound.

Management

• Borderline/mild disproportion — a closely-monitored trial of labour with a partograph, ready to proceed to caesarean section if progress arrests.
• Moderate-to-severe contraction or absolute CPD — elective caesarean section.
• Anticipate and prevent obstructed labour; never persist with augmentation against an unyielding pelvis.

WHY a 'trial of Labour' Is the Only Real Test

A subtle but vital concept is why the adequacy of a pelvis can rarely be decided in advance, and why the trial of labour is the true test. Clinical pelvimetry gives useful information (the diagonal conjugate, the sacral promontory, the ischial spines, the sub-pubic angle), and X-ray pelvimetry can measure the bones — yet neither predicts delivery reliably, because the outcome depends not on the pelvis alone but on the dynamic interaction of the three Ps: the passage (pelvis and soft tissues), the passenger (the size of the fetal head and, crucially, its ability to flex and mould) and the powers (the efficiency of the contractions). A pelvis inadequate for a large, deflexed, poorly-moulding head may be perfectly adequate for a smaller, well-flexed one. Hence, in borderline disproportion with a cephalic presentation and no other contraindication, the pelvis is 'tested' by allowing carefully-monitored labour — a trial of labour — with progress plotted on the partogram, watching descent, dilatation, caput and moulding; if progress is good, delivery follows, and if not, caesarean is performed before obstruction develops. Understanding that only labour itself can test the fit explains why a trial of labour is safe when supervised, and dangerous when it is not.

WHY Disproportion Matters Beyond This Delivery

Understanding why contracted pelvis is a public-health problem, not merely an obstetric one, gives the topic its proper weight in India. A contracted pelvis is largely a consequence of poverty and childhood undernutrition: rickets and osteomalacia (vitamin D deficiency), and stunting from chronic malnutrition, deform or simply shrink the pelvis — so it is commonest in exactly those women with the least access to skilled obstetric care. The consequences ripple outwards. In pregnancy it causes non-engagement of the head, malpresentation and an unstable lie; in labour, prolonged and obstructed labour, early rupture of the membranes with cord prolapse, uterine rupture and — in the survivors — obstetric fistula; and for the fetus, asphyxia, birth injury and death. Its prevention lies less in the labour room than in childhood nutrition and vitamin D, and in preventing early marriage and adolescent pregnancy (since a girl's pelvis is not fully grown until several years after menarche). Recognising that a contracted pelvis links a woman's childhood nutrition to her risk of dying in labour explains why it is as much a matter for public health as for the obstetrician.

• Cephalopelvic disproportion: the fetal head is too large, or the pelvis too small, for safe vaginal delivery.
• Causes of contracted pelvis: rickets/osteomalacia, malnutrition and stunting, trauma and congenital abnormality — largely diseases of poverty.
• Assess by clinical pelvimetry (diagonal conjugate, promontory, ischial spines, sub-pubic angle) and abdominally (head not engaged in a primigravida at term — Muller-Munro Kerr method).
• In borderline disproportion, a supervised trial OF labour on the partogram is the definitive test; abandon it for caesarean if progress fails.
• Absolute disproportion, or disproportion with a previous scar → elective caesarean section.