Obstetric Topic 20. Dystocia caused by abnormal presentation and position

I. Basic Concepts

Dystocia

• Dystocia: difficult or abnormally slow labor.
• Main causes: abnormalities of the "3 P's".
• Power: ineffective uterine contractions.
• Passage/pelvis: small or abnormal maternal pelvis.
• Passenger: abnormal fetal presentation, position, lie, attitude, size or structure.
• Normal for delivery: longitudinal lie + vertex presentation + flexed head + occipitoanterior (OA) position.

Definitions

• Lie: relation of fetal long axis to uterine long axis → longitudinal, transverse, oblique.
• Presentation: fetal part at pelvic inlet → vertex, breech, face, brow, shoulder, compound.
• Position: presenting part relative to maternal pelvis → OA, OP, OT, etc.
• Attitude: fetal head flexion/extension → vertex = flexed, brow = partial extension, face = full extension.
• Diagnosis: Leopold maneuvers + vaginal examination + ultrasound if uncertain.

II. Malpresentations

Breech Presentation

• Presenting part: buttocks or feet; incidence about 3-4% at term.
• Types: frank breech (hips flexed, knees extended), complete breech, incomplete/footling breech.
• Risk factors: prematurity, uterine anomalies/fibroids, fetal anomalies, placenta previa, multiple pregnancy, abnormal amniotic fluid.
• Risks: poor cervical dilating wedge → aftercoming head entrapment, cord compression/prolapse, birth trauma, intracranial hemorrhage.
• Management: offer external cephalic version (ECV) at term if no contraindication and cesarean service available.
• Persistent term breech: planned cesarean is common; planned vaginal breech only in selected cases with experienced team/protocol.
• Most favorable vaginal breech: frank breech, flexed head, normal pelvis, no fetal compromise/macrosomia.

Face Presentation

• Mechanism: full hyperextension of fetal head; presenting part = face, denominator = mentum/chin.
• Risk factors: multiparity, prematurity, fetal anomalies (anencephaly, neck mass/goiter), CPD.
• Diagnosis: vaginal exam; confirm with ultrasound if uncertain.
• Mentoanterior: vaginal delivery possible/expected if labor progresses.
• Mentoposterior: cannot deliver vaginally if persistent → cesarean section.

Brow Presentation

• Mechanism: partial extension of fetal head; presenting part = forehead.
• Risk factors: CPD, multiparity, prematurity, polyhydramnios.
• Often unstable → may convert to vertex or face.
• Persistent brow presentation: largest head diameter presents → cesarean section.

Shoulder Presentation / Transverse Lie

• Mechanism: fetal long axis perpendicular/oblique to maternal axis; shoulder or arm may present.
• Risk factors: multiparity, placenta previa, uterine anomaly/fibroid, multiple pregnancy, polyhydramnios, prematurity, pelvic tumor.
• Risks: cord prolapse, obstructed labor, uterine rupture.
• Before labor with intact membranes: consider ECV if no contraindication.
• In labor/persistent transverse lie: cesarean section.
• McRoberts + suprapubic pressure are for shoulder dystocia after head delivery, not for transverse lie itself.

Compound Presentation

• Definition: more than one presenting part, usually head + hand/arm.
• Risk factors: prematurity, polyhydramnios, multiparity, pelvic mass.
• Management: expectant if small fetal hand beside vertex and labor progresses; cesarean if persistent, cord prolapse, obstruction or fetal distress.

III. Malpositions

Persistent Occiput Posterior (OP)

• Occiput points toward maternal sacrum; common abnormal position.
• Most OP fetuses rotate spontaneously to OA during labor.
• Clinical: prolonged and painful second stage, back pain, caput/molding may obscure exam.
• Diagnosis: vaginal exam; ultrasound if uncertain.
• Management before/early second stage: expectant if progress and fetal heart rate are reassuring.
• Persistent OP with prolonged second stage: manual rotation by experienced clinician may be attempted.
• If rotation fails: operative vaginal delivery if criteria met, or cesarean section.

Persistent Occiput Transverse (OT)

• Occiput points toward maternal side wall.
• Normally enters pelvis as OT then rotates to OA; persistent OT causes transverse arrest.
• Causes: CPD, android/platypelloid pelvis, relaxed pelvic floor, epidural, multiparity.
• Diagnosis: sagittal suture/fontanelles in transverse diameter; ultrasound if exam unclear.
• Transverse arrest: persistent OT with arrest of descent for about ≥1 h.
• Management: expectant if descent continues and FHR reassuring; oxytocin if contractions inadequate and no CPD.
• No progress: manual/instrumental rotation by experienced clinician; cesarean if failed or unsafe.

IV. General Management Principles

1. Assess maternal pelvis, fetal size, presentation/position, membranes, contractions and fetal heart rate.
2. Correct power problem if safe: amniotomy/oxytocin for hypotonic labor.
3. Avoid vaginal delivery if obstructed labor, persistent transverse lie, persistent brow, persistent mentoposterior face, fetal distress or suspected CPD.
4. Use operative vaginal delivery only when cervix is fully dilated, head is engaged, position is known, pelvis is adequate and fetal status allows.
5. Cesarean section: indicated when safe vaginal delivery is unlikely or fetal/maternal compromise develops.