L19 RDA Physiology of parturition

Overview

This lecture covers the physiology of labour (parturition): the hormonal trigger for onset, the “3 Ps” framework (passage, passenger, power) used to describe how labour progresses, the stages of labour with their contraction patterns and cervical dilation, the fetal movements required to negotiate the pelvis, and the third stage (placental delivery).

Onset of labour

Labour onset is driven by three concurrent changes:

• Increase in prostaglandin synthesis
• Formation of myometrial gap junctions (allows coordinated contraction between myometrial cells)
• Up-regulation of myometrial oxytocin receptors

These feed into a positive feedback loop:

1. Estrogen (from the ovaries) induces oxytocin receptors on the uterus.
2. Oxytocin (from the fetus and the mother’s posterior pituitary) stimulates the uterus to contract and stimulates the placenta to produce prostaglandins.
3. Prostaglandins stimulate more vigorous uterine contractions.
4. Increased contractions further increase oxytocin release, closing the loop (positive feedback).

The 3 Ps framework

Labour and its progress are described using three factors:

• Passage - the pelvis (bony) and cervix (soft tissue)
• Passenger - the fetus (position and degree of flexion)
• Power - the uterine contractions

Passage: the pelvis

Two pelvis shapes are compared:

• Gynecoid (A) - circular pelvic inlet, pubic arch angle 80-85° (favourable for vaginal birth)
• Android (B) - heart-shaped pelvic inlet, prominent medially projecting ischial spines, prominent projecting sacral promontory, pubic arch angle 50-60°

Station describes the level of the fetal vertex relative to the ischial spines:

• Station 0 = fetal vertex at the level of the ischial spines
• Scale runs from -3 (above the spines) to +3 (below the spines)

Passage: the cervix

Cervical change in labour is described by effacement and dilation:

• A: cervix not effaced or dilated
• B: cervix fully effaced, dilated to 1 cm
• C: cervix dilated to 5 cm
• D: cervix fully dilated to 10 cm

Dilation = the gradual opening of the cervix, measured in cm from 0 to 10.

Power: contractions and stages of labour

Labour proceeds through defined phases, each with a characteristic contraction pattern and degree of cervical dilation:

Phase	Stage	Duration	Contraction length/interval	Frequency	Dilation
Early (latent)	1st	7-8 hours	~30 sec / 8 min interval	every 5-30 min	0-3 cm
Active	1st	3-5 hours	60 sec / 2 min interval	every 3-5 min	3-7 cm
Transition	1st	0.5-1.5 hours	70-90 sec / 1 min interval	every 2.5-3 min	7-10 cm
Pushing to birth	2nd	0.5-2 hours	60 sec / 3 min interval	every 3-5 min	10 cm to birth

The overall stages of labour are:

1. Quiescence
2. Latent phase - effacement of the cervix
3. Active 1st stage - cervix dilates to 10 cm
4. 2nd stage - full dilatation to delivery of the fetus
5. 3rd stage - delivery of the placenta

Passenger: fetal flexion and presentation

Degree of fetal head flexion determines the presenting diameter at the pelvic inlet (from most to least favourable):

• Vertex presentation - complete flexion, smallest presenting diameter (most favourable)
• Military presentation - moderate flexion
• Brow presentation - poor flexion (extension)
• Face presentation - full extension, largest/least favourable presenting diameter

Passenger: fetal position

Fetal head position is described relative to the maternal pelvis using the occiput as the reference point:

• Right occiput anterior (ROA)
• Right occiput transverse (ROT)
• Right occiput posterior (ROP)
• Left occiput anterior (LOA) - most favourable position
• Left occiput transverse (LOT)
• Left occiput posterior (LOP)

Mechanism of labour: cardinal movements

The fetus negotiates the pelvis through a fixed sequence of movements:

1. Engagement
2. Descent
3. Flexion
4. Internal rotation
5. Extension
6. External rotation (restitution, then shoulder rotation)
7. Expulsion

Third stage: placental delivery

The third stage is the delivery of the placenta following birth of the fetus. The slides show the maternal and fetal surfaces of a delivered placenta with the umbilical cord attached, and the immediate post-birth period with the newborn and placenta/cord.

Self-test

1. Name the three hormonal/tissue changes that occur at the onset of labour.
2. Describe the steps of the positive feedback loop that drives progression of labour, starting from estrogen’s action on the uterus.
3. What are the “3 Ps” of labour, and what does each refer to?
4. Distinguish a gynecoid from an android pelvis in terms of pelvic inlet shape and pubic arch angle.
5. Define fetal station, including what station 0 means and the range of the scale.
6. Define cervical dilation and describe how it progresses from A to D on the effacement/dilation diagram.
7. Describe how contraction duration, interval, and cervical dilation change from the early phase through to the second stage of labour.
8. List the four fetal presentations in order of increasing extension, and state which is most favourable and why.
9. List the six fetal positions described by occiput orientation, and state which is the most favourable.
10. Describe the sequence of cardinal movements the fetus undergoes during labour.
11. What occurs during the third stage of labour?

Answers

Reveal answers

1. Increase in prostaglandin synthesis, formation of myometrial gap junctions, and up-regulation of myometrial oxytocin receptors.
2. Estrogen induces oxytocin receptors on the uterus; oxytocin (from the fetus and maternal posterior pituitary) stimulates uterine contraction and placental prostaglandin production; prostaglandins increase contraction vigour; increased contractions further increase oxytocin release, reinforcing the loop.
3. Passage (the pelvis and cervix), passenger (the fetus), and power (uterine contractions) - the three factors that determine how labour progresses.
4. Gynecoid: circular pelvic inlet, pubic arch angle 80-85° (favourable). Android: heart-shaped pelvic inlet with prominent ischial spines and sacral promontory, pubic arch angle 50-60° (less favourable).
5. Station describes the level of the fetal vertex relative to the ischial spines; station 0 is when the vertex is level with the spines; the scale runs from -3 (above the spines) to +3 (below the spines).
6. Dilation is the gradual opening of the cervix measured in cm from 0 to 10. A: not effaced or dilated; B: fully effaced, dilated to 1 cm; C: dilated to 5 cm; D: fully dilated to 10 cm.
7. Early phase: ~30 sec contractions every 5-30 min, 0-3 cm dilation. Active phase: 60 sec contractions every 3-5 min, 3-7 cm. Transition phase: 70-90 sec contractions every 2.5-3 min, 7-10 cm. Second stage (pushing to birth): 60 sec contractions every 3-5 min, 10 cm to birth. Contractions get longer and more frequent, and dilation increases, as labour progresses.
8. Vertex (complete flexion, most favourable, smallest presenting diameter) > military (moderate flexion) > brow (poor flexion/extension) > face (full extension, largest/least favourable diameter). Vertex is most favourable because complete flexion presents the smallest fetal head diameter to the pelvis.
9. Right occiput anterior (ROA), right occiput transverse (ROT), right occiput posterior (ROP), left occiput anterior (LOA), left occiput transverse (LOT), left occiput posterior (LOP). Left occiput anterior (LOA) is most favourable.
10. Engagement, descent, flexion, internal rotation, extension, external rotation (restitution, then shoulder rotation), expulsion.
11. Delivery of the placenta following birth of the fetus.