Obstetric Topic 57. The mechanism of normal labor

I. Normal Labor

Definition

• Labor: physiologic process of regular uterine contractions → cervical effacement and dilatation → fetal delivery through birth canal.
• Clinical labor: painful regular contractions causing cervical change.
• Presentation: low back/abdominal pain, regular contractions, rupture of membranes or bloody show.
• Bloody show: mucus plug + small blood loss after cervical effacement/dilatation.

Criteria of Normal Birth

1. Spontaneous onset and expulsion.
2. Single mature fetus: 37-42 weeks.
3. Longitudinal lie, vertex presentation.
4. Vaginal delivery through birth canal.
5. Reasonable duration and no maternal/fetal complications.

Cervical Changes

• Cervical ripening: ↑ water content + collagen remodeling/lysis, partly prostaglandin-mediated.
• Effacement: cervix shortens/thins as it is taken into the lower uterine segment.
• Dilatation: cervical opening widens to 10 cm.

II. Stages of Labor

Stage I: Cervical Dilatation

• Stage I: onset of labor → complete cervical dilatation (10 cm).
• Latent phase: 0 → about 6 cm; slow/variable progress.
• Active phase: about 6 → 10 cm; faster dilatation and descent.
• Active-phase arrest: no cervical change at ≥6 cm with ruptured membranes despite adequate contractions/time.

Stage II: Fetal Expulsion

• Stage II: complete dilatation → birth of fetus.
• Phases: propulsive descent → expulsive pushing.
• Prolonged 2nd stage: commonly >3 h pushing in nullipara or >2 h in multipara; individualize by descent, rotation, epidural and fetal status.

Stages III-IV

• Stage III: fetal expulsion → placental expulsion.
• Mechanism: placental separation → expulsion.
• Stage IV: first 2 h after placental delivery → uterine tone, hemostasis and beginning involution.

III. Fetal Orientation

Lie, Presentation, Attitude

• Fetal lie: relation of fetal long axis to maternal long axis: longitudinal, transverse or oblique.
• Presentation: fetal part over pelvic inlet.
• Cephalic/vertex is normal; breech = buttocks/feet; shoulder usually means transverse/oblique lie.
• Attitude/posture: relation of fetal parts to each other.
• Normal attitude: flexed head, flexed hips, bent knees, convex back → ovoid shape matching uterus.

Position and Station

• Fetal head position: relation of occiput/fontanelles/sagittal suture to maternal pelvis on vaginal exam.
• Occiput anterior: most favorable for normal mechanism; OP/transverse positions may slow rotation/descent.
• Fetal station: descent of presenting bony part relative to ischial spines.
• Station 0: presenting part at ischial spines; negative above, positive below.
• Engagement: widest transverse diameter of presenting part has passed pelvic inlet; often around station 0 in vertex presentation.

IV. Cardinal Movements

Mechanism of Vertex Delivery

1. Engagement: biparietal diameter enters pelvic inlet.
2. Descent: head moves down through pelvis with contractions and maternal pushing.
3. Flexion: chin moves toward chest → smaller suboccipitobregmatic diameter presents.
4. Internal rotation: occiput rotates anteriorly toward symphysis pubis.
5. Extension: head extends under symphysis and delivers through the outlet.
6. Restitution / external rotation: delivered head rotates to realign with fetal shoulders.
7. Expulsion: anterior shoulder under symphysis → posterior shoulder over perineum → body delivers.

Memory Aid

• Mnemonic: Every Darn Fool In Egypt Eats Raw Eggs.
• Engagement → Descent → Flexion → Internal rotation → Extension → Restitution/external rotation → Expulsion.

V. Practical Exam Points

Normal vs Dysfunctional Labor

• Normal labor: progressive cervical dilatation + fetal descent + reassuring maternal/fetal condition.
• Dysfunctional labor: progress of dilatation or descent is slower than expected or stops.
• Always assess 3 P's: powers, passenger, passage.
• Powers: contractions and pushing.
• Passenger: lie, presentation, position, attitude, size, anomalies.
• Passage: pelvis and soft tissues.
• Do not diagnose active-phase arrest before active labor (about 6 cm) unless another emergency indication exists.