Cephalopelvic Disproportion

01Pathophysiology

The fundamental concept behind cephalopelvic disproportion is a mechanical obstruction to vaginal delivery. For a fetus to pass through the birth canal, three variables must align: the power (uterine contractions), the passenger (fetal size, lie, presentation, and attitude), and the passage (bony pelvis and soft tissues). CPD occurs when the passenger is too large relative to the passage, regardless of how strong the power is.

The maternal bony pelvis has three critical planes. The pelvic inlet (obstetric conjugate, normally >10 cm) is where engagement occurs. The midpelvis (interspinous diameter, normally >10 cm) is the narrowest fixed bony dimension and the most common site of arrest. The pelvic outlet (intertuberous diameter, normally >8 cm) rarely causes obstruction alone. Contracture at any of these levels can prevent descent.

On the fetal side, the key measurement is the biparietal diameter (BPD), which averages 9.5 cm at term. Macrosomic fetuses, hydrocephalic fetuses, or fetuses in deflexed attitudes (brow or face presentation) present larger diameters to the pelvis, creating disproportion even in a normal-sized pelvis. A persistent occiput posterior position, for example, presents the occipitofrontal diameter (11.5 cm) instead of the smaller suboccipitobregmatic diameter (9.5 cm), effectively creating a functional CPD.

The clinical consequence of this mismatch is arrest of labor. Despite adequate contractions, the cervix stops dilating (arrest of dilation) or the fetal head fails to descend through the pelvis (arrest of descent). Prolonged obstruction can lead to uterine rupture, fetal hypoxia, and obstetric fistula formation if not recognized and managed.

02Classification and Clinical Manifestation

Pelvic Types (Caldwell-Moloy Classification)

Arrest Disorders in Labor

Fetal Malpresentations Contributing to Functional CPD

03Diagnostic Workup

CPD is fundamentally a clinical diagnosis made retrospectively during labor. There is no single test that definitively predicts CPD before a trial of labor. This is a heavily tested concept: the exam will present a patient in active labor who is not progressing and ask you to determine why.

The first step when labor arrests is not to diagnose CPD immediately. Instead, you must first determine whether the contractions are adequate. An IUPC is placed to measure Montevideo units. If contractions are inadequate (<200 MVU), the problem is the power, not the passage, and you augment with oxytocin. Only after you have confirmed adequate contractions (>200 MVU for at least 2-4 hours) with no cervical change can you label the arrest as CPD.

Clinical pelvimetry during prenatal visits can raise suspicion. If the diagonal conjugate (measured from the lower border of the symphysis to the sacral promontory) is <11.5 cm, the obstetric conjugate (which is about 1.5 cm shorter) may be <10 cm, suggesting inlet contracture. However, clinical pelvimetry alone is not sufficient to recommend a primary cesarean. A trial of labor is still indicated unless there is an absolute contraindication.

Ultrasound for estimated fetal weight is important but carries a margin of error of 15-20%. An EFW of 5000 g in a non-diabetic mother or 4500 g in a diabetic mother is considered a threshold where elective cesarean may be offered without a trial of labor. Below these thresholds, suspected macrosomia alone is not an indication for cesarean.

04Management and Treatment

When a patient in active labor shows no progress, the first intervention is to assess contraction adequacy. Place an IUPC if external monitoring is ambiguous. If MVU is <200, begin oxytocin augmentation. Standard low-dose protocol begins at 0.5-2 mU/min and increases by 1-2 mU/min every 30-40 minutes, titrating to achieve 3-5 contractions per 10 minutes with MVU >200. Amniotomy (artificial rupture of membranes) can also be performed simultaneously to augment labor.

If contractions are adequate (MVU >200) and the cervix has not changed for 4 hours, or if contractions remain inadequate after maximum oxytocin and 6 hours have passed without cervical change, the diagnosis is arrest of active phase, and the next step is cesarean delivery. Continuing oxytocin beyond this point increases the risk of uterine rupture, chorioamnionitis, and fetal distress without meaningful benefit.

In the second stage, if descent has arrested for the defined time threshold (2-3 hours depending on parity and epidural status), you must decide between operative vaginal delivery and cesarean. Operative vaginal delivery (vacuum or forceps) is only appropriate if the fetal head is at station +2 or below, there is no suspected CPD at the midpelvis level, and the operator is experienced. If the station remains high (0 or +1), this strongly suggests true CPD, and cesarean is indicated. Attempting forceps at a high station ("high forceps") is contraindicated due to the risk of catastrophic maternal and fetal injury.

Contraindications to note: Oxytocin is contraindicated in the setting of prior classical (vertical) uterine incision, as the risk of uterine rupture is too high. In a patient with a prior low transverse cesarean, oxytocin may be used cautiously during TOLAC but requires continuous fetal monitoring.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common trap on this topic is diagnosing CPD prematurely before confirming contraction adequacy. The vignette will describe a patient with arrested dilation and ask for the next step. Students who jump straight to cesarean delivery will get the question wrong. The correct answer is almost always to assess or augment contractions first. You cannot call it CPD until the power has been proven adequate and labor still fails to progress.

A second common trap involves estimated fetal weight. A vignette may mention an EFW of 4200 g and tempt you to recommend cesarean for macrosomia. However, in a non-diabetic mother, the threshold for elective cesarean based on weight alone is 5000 g, not 4000 g or 4500 g. Below that threshold, a trial of labor is appropriate regardless of the estimated weight.

Third, remember that clinical pelvimetry and prenatal ultrasound cannot reliably predict CPD. The exam may offer a distractor like "schedule elective cesarean due to small pelvis on clinical exam." This is incorrect. The only reliable way to diagnose CPD is through a trial of labor with documented adequate contractions and failure to progress.

The core competency being tested is your ability to sequentially troubleshoot labor arrest: first rule out inadequate power, then confirm a true mechanical obstruction, and only then proceed to cesarean delivery. The test-writer wants to see that you understand the algorithm, not just the diagnosis.