Normal labour is the physiological process by which the fetus, placenta, and membranes are expelled from the uterus through the birth canal after the pregnancy reaches viability.
It is a complex interplay of uterine contractions, pelvic anatomy, and fetal movements.
For an obstetrician, midwife, or medical student, understanding the mechanism of labour is essential for anticipating complications and assisting delivery effectively.
Definition of Normal Labour
Labour is considered normal when it meets these criteria:
- Spontaneous onset at term (37–42 weeks gestation)
- Vertex presentation
- No maternal or fetal complications
- Progresses without need for instrumental or surgical intervention
- Results in delivery of a healthy baby and placenta
Stages of Labour
Labour is classically divided into three stages:
First Stage
- From onset of regular painful uterine contractions to full cervical dilatation (10 cm).
- Latent phase: Cervix dilates slowly up to 4 cm.
- Active phase: Rapid dilatation from 4 cm to 10 cm.
Second Stage
- From full cervical dilatation to delivery of the baby.
- This is where the mechanism of labour plays a central role.
Third Stage
- From delivery of the baby to expulsion of the placenta and membranes.
Mechanism of Labour – 9 Steps
The mechanism refers to the cardinal movements the fetus undergoes to navigate through the maternal pelvis.
1. Engagement
- Definition: The largest transverse diameter of the fetal head (biparietal diameter) passes through the pelvic inlet.
- Clinical note: In primigravidas, engagement usually occurs before labour; in multigravidas, it may occur during early labour.
2. Descent
Downward movement of the fetal head through the pelvis due to:
- Uterine contractions
- Maternal pushing
- Straightening of fetal body
3. Flexion of Head
- Fetal chin moves towards chest, presenting the smallest diameter (suboccipitobregmatic) to the birth canal.
- Facilitates easier passage.
4. Internal Rotation of Head
- The occiput rotates anteriorly towards the maternal symphysis pubis.
- Aligns the head with the widest anteroposterior pelvic outlet diameter.
5. Crowning
- The widest diameter of the fetal head stretches the vulval ring and remains visible between contractions.
- Indicates imminent delivery of the head.
6. Delivery of Head by Extension
- Occiput pivots under the pubic symphysis, and the head extends to deliver the face and chin.
- Requires gentle support to avoid perineal tears.
7. Restitution
- After delivery of the head, it realigns with the fetal shoulders inside the pelvis.
- Head rotates slightly to the side.
8. External Rotation of Head
- Further rotation of the head aligns with the shoulders’ position.
- Prepares for delivery of the shoulders.
9. Delivery of Baby’s Shoulder & Body
- Anterior shoulder delivered first under pubic symphysis.
- Posterior shoulder follows with gentle traction.
- Rest of the body slips out smoothly.
Physiological Basis of the Mechanism
- Maternal factors: Uterine contractility, pelvic dimensions, soft tissue elasticity.
- Fetal factors: Head size, degree of moulding, lie, and presentation.
- Gravity: Aids descent, especially in upright positions.
Clinical Importance
- Knowing the steps allows early recognition of labour dystocia (obstructed labour).
- Deviations (e.g., persistent occiput posterior position) may require interventions such as manual rotation, vacuum, or forceps.
- Understanding helps in perineal protection techniques to reduce obstetric trauma.
Common Complications if Steps Fail
| Step | Possible Complication |
|---|---|
| Engagement | Non-engagement → Cephalopelvic disproportion |
| Descent | Arrest of descent |
| Flexion | Deflexed head → Brow/face presentation |
| Internal rotation | Persistent occiput posterior/transverse position |
| Crowning | Perineal tears if not supported |
| Delivery by extension | Shoulder dystocia |
| Restitution/External rotation | Rotational misalignment |
| Shoulder delivery | Brachial plexus injury if excessive force applied |
Quick Reference – 9 Steps of Normal Labour Mechanism
| Step No. | Name |
|---|---|
| 1 | Engagement |
| 2 | Descent |
| 3 | Flexion of head |
| 4 | Internal rotation of head |
| 5 | Crowning |
| 6 | Delivery of head by extension |
| 7 | Restitution |
| 8 | External rotation of head |
| 9 | Delivery of baby’s shoulder & body |
FAQs
1. Is the mechanism of labour the same in all deliveries?
No, variations occur based on fetal position, pelvic shape, and maternal factors.
2. Which step indicates that delivery is very close?
Crowning — when the head remains visible between contractions.
3. Can normal labour happen without all steps occurring exactly?
Yes, but deviation from the normal sequence may require assistance.
4. How can a mother help labour progress?
By maintaining mobility, upright posture, and effective pushing during the second stage.
5. Is pain relief (epidural) safe during normal labour?
Yes, when administered by trained professionals, but it may slightly prolong the second stage.
